Lou Godefroy
email: l.godefroy@vu.nl

Aurélie Salavert

Evi Margaritis

Enlightening the darkness: The potential of charcoal analysis for the study of vegetation dynamics and relative exploitation at Krania during the Iron Age Greece

Keywords: Anthracology, Greek colonization, Iron Age, Ancient Greece, Macedonia, Krania

. As a part of the subproject 'Flora & Fauna', which focuses on farming practices, subsistence strategies and surplus production, Iron Age material from the settlement of Krania in the Pieria region of Greece was analyzed. The site was excavated in 2001 and revealed occupation phases from the 3rd millennium BC to the Hellenistic period. The amount of ceramics recovered demonstrates the importance of trade with the rest of Greece, in particular Corinth, providing evidence for long-standing contacts and networks. Two pit structures contained a high concentration of material, notably charcoal and ceramics dating to the 9 th -8 th centuries BC. This material constitutes an interesting case study in itself but also as a regional comparandum with other Greek assemblages. This paper discusses how anthracological analysis can contribute to our understanding of human-environment interactions, anthropogenic practices and environmental management.

Introduction and background

During the transition from the Bronze Age to the Iron Age, a decrease in regional contact is noticeable in Greece. Archaeological evidences such as craft production, pottery and jewellery attest some contact between the South and the North regions (Gimatzidis et al. 2018[START_REF] Gimatzidis | The economy of early Greek colonization in the northern Aegean[END_REF]). However, during the 9 th century BC, signs in the recovery of regional exchange are noticeable, suggesting an increase of contact between the Southern Aegean, Italy and the Northern Aegean [START_REF] Broodbank | The Making of the Middle Sea, A History of the Mediterranean from the Beginning to the Emergence[END_REF]. In this new dynamic period, migrants from the Aegean started to settle along the shores of the entire Mediterranean basin. Among the reasons for this migration were the favourable opportunities for economic and social expansion, permitted by territories that were not prior occupied by Greek communities. The necessity to adapt to new lands and develop relevant practices, raises the question of the migrants' ability to manage within new environments and carry out different agricultural practices. However, agriculture and exploitation of natural resources have not been included in the present debate about the so-called Greek colonization movement, despite the fact that studying its organization during the initial phase of the Greek migrations to Southern Italy is an important aspect when approaching these ancient communities [START_REF] Renfrew | The Emergence of Civilisation : the Cyclades and the Aegean in the Third Millenium B.C[END_REF][START_REF] Foxhall | Bronze to iron: agricultural systems and political structures in late bronze age and early iron age Greece[END_REF], 2015). Over the last decades, agricultural practices and subsistence economies were generally not considered to be a decisive factor in understanding the social, political, and economic aspects of these ancient communities.

Examining subsistence helps us to better understand the level and degree of interactions between indigenous and allochthonous populations. It also permits to evaluate chronological and spatial continuities or discontinuities in economic activities. Finally, it allows us to detect Greek influences in autochthonous settlements and vice-versa as it directly affects the primary and fundamental stages of the development of societies [START_REF] Crielaard | What went into the Melting Pot? Land use, agriculture, and craft production as indicators for the contributions of Greek migrants and local inhabitants to the so-called Greek colonization in Italy (800-550 BC)[END_REF]. The confrontation of subsistence regimes and strategies over time and space in Greece and Southern Italy is a complex approach that needs to be considered stepwise. Three different geographical scales are defined in the frame of the 'Flora&Fauna' project: microscale (site level and case studies), macroscale (results from the case studies in a broader spatial, diachronic and cultural contexts) and megascale (confrontation of the macroscales observations).

Unfortunately, limited archaeobotanical evidence has hampered the understanding of the socio-economic role of primary goods. Although the study of macroremains has helped to counterbalance this lack of knowledge, anthracology can be applied, especially to approach landscape reconstructions and their related land use. Although charcoal is collected more frequently in excavations these days, anthracology applied to the 1 st millennium BC is still developing in Greece and the material is usually kept in storage and seldomly studied [START_REF] Lentjes | A walk in the woods. Charcoal analysis from three Greek archaeological sites: Geraki, Plakari and Titane[END_REF]. Only four settlements have been analysed thus far for Iron Age Greece: Delphi (Renault-Miskovsky and Thiébault 1997), Plakari [START_REF] Lentjes | A walk in the woods. Charcoal analysis from three Greek archaeological sites: Geraki, Plakari and Titane[END_REF], Corinth [START_REF] Bookidis | Dining in the sanctuary of Demeter and Kore at Corinth[END_REF], and Krania (in this connection, the island of Crete is not considered due to its strong ecological and cultural differences with the rest of Greece). A fair amount of palynological work in Greece has been applied to prehistoric contexts but few of these are specifically aimed at reconstructing the direct environment of ancient settlements [START_REF] Lentjes | A walk in the woods. Charcoal analysis from three Greek archaeological sites: Geraki, Plakari and Titane[END_REF]. While it is perilous to reconstruct the natural environment of settlements based solely on charcoal datasets without including other proxy data such as pollen, charcoal analyses can provide interesting insights for landscape reconstructions and their related land use. Through charcoal analyses, it is possible to obtain an idea of the local signal of the vegetation as well as to make an estimation of species selection, woodcutting, and/or clearance practices [START_REF] Chabal | L'anthracologie[END_REF][START_REF] Asouti | Reconstructing woodland vegetation and its exploitation by Past Societies, based on the Analysis and Interpretation of Archaeological wood charcoal macro-remains[END_REF][START_REF] Chabal | Sampling and quantitative analysis methods in anthracology from archaeological contexts: achievements and prospects[END_REF]. This paper focuses on the local environment of the Krania settlement. Krania's charcoal represent a part of the microscale approach within the "Flora&Fauna" project. This original study, by allowing us to examine the use of wood ressources in the local environment of the settlement, constitutes an important case study, independent on its own, but also significant to enrich macro-and megascales corpus. Combining charcoal assemblages with the pollen record allows us to determine various aspects of the ecological milieu (open areas, potential fruit growing strategies, landscape reconstructions, land use and resource accessibility, [START_REF] Lentjes | A walk in the woods. Charcoal analysis from three Greek archaeological sites: Geraki, Plakari and Titane[END_REF]. Through this, we can create a more complete picture of the exploitation strategies in Iron Age Greece.

2.The Site of Krania

Location and environmental setting of the area

The environmental background of the studied area significantly determines the taxa distribution and availability, influencing the potential character of woodland management and cultivation practices (Shackleton andPrins 1992, Delhon 2021). This is true for today as well as for the past.

The Pieria region, situated in the southern part of Macedonia (fig. 1), is surrounded by the Aliakmon River to the north and the Peneios River delta in the south (Hammond 1972, 123;Margaritis 2005, 66).

The plain of Kateríni, opening to the Aegean Sea on the east, is one of the largest and most fertile areas of Greece and forms the center of Pieria. In the area along the Thermaic Gulf, a narrow coastal strip separates the Pieria and the Olympus massif from the Aegean. The site of Krania is located in the lowland of Pieria, in a fertile passage connecting Macedonia with Thessaly. The inland part of Pieria is a terraced land, deeply cut by many streams, with passes connecting the lowlands and the mountainous areas (Poulaki 2013, 21;Margaritis 2005, 67).

The temperatures in this region are generally lower than the typical Mediterranean climate that easily goes beyond 35°C in summer [START_REF] Polunin | Flowers of Greece and the Balkans, a Field Guide[END_REF]King et al. 1997). The mean annual rainfall is between 500-750 mm (Semple 1932, 86;Anathasiadis 1975;Polunin 1987, 13;Margaritis 2005, 67).

The foothills of Mount Olympus, where Pieria is located, are composed of marls, schists, volcanic lavas, and other rocks (Polunin 1987, 2;Margaritis 2005, 67;[START_REF] Gerasimidis | Five decades of rapid forest spread in the Pieria Mountains (N. Greece) reconstructed by means of high-resolution pollen analysis and aerial photographs[END_REF]. The main types of vegetation in the Balkan Peninsula are divided according to altitude zones: lowlands and hills (app. 0-700m), montane forests (app.700-1700 m), subalpine and alpine (app. 1700-3000m). The studied site lies within the lowland zone where Mediterranean plant communities are dominant, characterized by the prevalence of evergreen trees, shrubs and small aromatic shrubs [START_REF] Strid | Wild Flowers of Mount Olympus. -Kifissia: Goulandris Natural History Museum[END_REF][START_REF] Polunin | Flowers of Greece and the Balkans, a Field Guide[END_REF] 1975). The present-day vegetation in the vicinity of the settlement has been highly influenced by anthropogenic activities, causing rapid changes in vegetation structure and composition. The anthracological remains can be perceived as remnants of the ancient vegetation, from both regional and local hotspots that still comprise a high tree richness (including 146 native including endemics, [START_REF] Quézel | Écologie et biogéographie des forêts du bassin méditerranéen[END_REF][START_REF] Médail | Mediterranean[END_REF][START_REF] Médail | What is a tree in the Mediterranean Basin hotspot? A critical analysis[END_REF]).

Archaeological background of Krania -ancient Irakleon

The settlement of Krania was excavated in 2001 under the direction of E. Poulaki-Pandermali, under the auspices of the Euphorate of Antiquities of Pieria, as part of the construction work for the new Athens-Thessaloniki railway. The site is situated at the northern foothill of Platamon Hill, 200 meters from the sea and at 10 meters above sea level [START_REF] Margaritis | Archaeobotanical investigations at the geometric site of Krania, Southern Pieria in Macedonia, Greece[END_REF]. Its particular location, on the lower Olympus, provides Krania with a strategic position above the Tempi valley (Poulaki 2003). Krania was likely a part of the harbor district of the ancient city of Irakleon [START_REF] Poulaki-Pantermali | Έργα εθνικά και αλλά στην περιοχή του Μακεδονικού Ολύμπου[END_REF][START_REF] Margaritis | Archaeobotanical investigations at the geometric site of Krania, Southern Pieria in Macedonia, Greece[END_REF].

Studies of the stratigraphy and archaeological finds revealed a discontinuous occupation from the 3 rd millennium BC to the 4 th century AD, with several phases of destruction. A Bronze Age monumental peribolos (stone enclosure) was destroyed by a fire during the Early Iron Age. The settlement was destroyed again during the 8 th century BC and covered by a homogeneous layer of destruction. Habitation consisted of orthogonal buildings, terraces, and peribolos walls, as well as apsidal buildings. Within these, finds relating to the daily life were preserved such as domestic utensils, loom weights, and vessels used for various purposes, in addition to animal bones and clams (fig.

2,3).

The archaeobotanical material, both seeds and charcoal, are dated to the 9 th and 8 th centuries BC, i.e., towards the end of the site's Iron Age occupation. The samples come from two structures that were part of the habitation area on the slope of the hill. The first pit, Pit A (approx. 2 m x 1.5 m in size and one meter in depth), contained large amounts of amphorae and other storage vessels that were in situ but broken. Charred plant remains and charcoals were visible on these fragments. Pit B, which is larger than Pit A and ellipsoid in shape, had off-white coating and contained vessels as well as two storage amphorae next to a brazier, an iron axe, nails, and other types of vases [START_REF] Poulaki-Pantermali | Έργα εθνικά και αλλά στην περιοχή του Μακεδονικού Ολύμπου[END_REF][START_REF] Margaritis | Archaeobotanical investigations at the geometric site of Krania, Southern Pieria in Macedonia, Greece[END_REF]). The two pits do not seem to be connected to any building. The pottery fragments are characteristic of Corinthian production and suggests contacts and potential trade between the inhabitants of Krania and areas in the Southern Aegean. In addition to these Corinthian imports, there is pottery that belongs to the Thessalo-Euboean koine

(Poulaki-Pantermali 2013).
The two pits have yielded one of the largest assemblages of plant macroremains in Early Iron Age Greece with the presence of Sesamum indicum which suggests long distance trades with the Eastern Mediterranean (Godefroy and Margaritis, in preparation).

Following up on E. Margaritis' preliminary analysis of 2007, for the present study almost 10 000 macroremains were analyzed (Godefroy and Margaritis, in preparation). Based on field information and preliminary archaeobotanical analysis, it appears that the mode and type of filling is different between the two pits. Pit A is interpreted as a burned underground grain storage structure, while pit B could be a detrital pit composed of various episodes of domestic refuse (Margaritis 2017, 684). Moreover, the assemblages are unique since they provide solid evidence for the exploitation of the settlement's local environment.

Material and method

The two studied pits were sampled for organic remains. The volume of soil collected for pit B was between 12-18 liters. In Pit A, the deposit consisted of large quantities of charred plant remains, visible during the excavation, for a minimum of 86 liters (Margaritis 2017, 124). Each pit has been sampled using the same method: the filling has been subsampled and documented according to the type of material and deposition units.

The sediment samples from the site were processed by E. Margaritis with the help of a flotation machine. The sieves used during flotation were of two mesh sizes: 1 mm for the coarse flot and 300µ for the fine flot. The riddle retaining the 'heavy residue' of the samples was of 1mm mesh size.

In the laboratory, charcoal were separated from the rest of the organic fraction using a 4 mm sieve which enabled isolation of the biggest fractions from the rest. The pieces bigger than 4 mm were then identified under an Olympus BX51M microscope with a magnification of 50x-500x. For the identification, anatomic criteria were used and compared with atlas (Jacquiot 1955, Schweingruber 1990, fig. 4) and the charcoal reference collection of the AASPE laboratory at the National Museum of Natural History in Paris. The wood charcoal recovered at Krania were well preserved, although there was a high rate of fragmentation. The charcoal fragments were individually counted.

Results

General remarks

A total of 269 fragments were analyzed to obtain a representative picture of Krania's past environment. The archaeological study of the pits is not complete and therefore, not all charcoal samples could be linked to the stratigraphical data. Of 123 samples, 83 were further analyzed because of their correspondence with stratigraphic descriptions. However, of these 83 samples, only 12 samples with charcoal remains could be linked to either Pit A or Pit B.

Pit A

A total of 11 charcoal samples and 197 fragments were analyzed, which constituted the biggest dataset of the two structures. Fourteen taxa were identified. The pit assemblage is dominated by oak (Quercus spp., 25%), including deciduous and evergreen types, as well as pine (Pinus spp., 24 %, tab. 1). Pinus cf. sylvestris/nigra has been identified based on fenestriform pits and dentate tracheid walls present in the cross-fields. In addition, P. cf.

peuce was discriminated based on the small areolate punctuation on the tracheid walls in the final wood, on the presence of two pits per cross-fields but also on its prevalence in Macedonia since no anatomic distinction can be observed between P. cembra, P. peuce and P. strobus (Jacquiot 1955, Shwreingruber 1990). Both taxa (Quercus spp., and Pinus spp.) are present in most of the samples collected from this pit. The secondary taxa are olive (Olea europaea), ash (Fraxinus), maple (Acer), and Cornus/Cornus mas (between 5 and 15% each). Cornelian cherry (Cornus mas) was identified thanks to its characteristic anatomical criteria, such as less distinct growth ring boundaries, fewer pores, and thickerwalled fibre tracheids in comparison to common dogwood (Cornus sanguinea). Regarding Cornus, the remains are from one sole sample, suggesting an overrepresentation of the fragments for this taxa in this pit. The distribution of other secondary taxa is overall homogeneous in this structure, found together with a concentration of pottery. Finally, plane (Platanus), Phillyrea/buckthorn (Phillyrea/Rhamnus), common hornbeam (Carpinus), Cercis, strawberry tree (Arbutus unedo) and alder (Alnus) constitute between 4 and 0,5 % of the assemblage.

Pit B

During the analyses of bag NK280, which was the single documented sample with charcoal for this structure, 72 pieces of charcoal were studied. Thirteen taxa have been identified; the assemblage is dominated by maple (24%), oak (23%) and pine (14%, tab.1).

The secondary taxa identified are Cornus/Cornus mas as well as olive and ash (between 5 and 9% each). Besides, strawberry tree (Arbutus unedo), elm (Ulmus), Phillyrea/buckthorn (Phillyrea/Rhamnus), Cercis and Maloideae were identified each between 4 and 1% of the assemblage.

Comparison of the two pits

The comparison of the anthracological assemblages of the two pits must be made with caution. The number of samples and relative wood pieces are higher in Pit A than in Pit B which accompany the highest range of species diversity. In fact, Pit B, is represented by a single sample with a restraint taxa diversity that brings an over representation of some taxa in the assemblage. For example, in Pit A, Quercus spp. together with Pinus spp. dominate whereas Acer together with Cornus are secondary. For pit B, Acer dominates with Quercus spp. but Pinus spp. is secondary (see relative frequencies in tab. 1). In any case, the similarities between both pits in terms of main taxa and equivalent taxonomical diversity suggest the same filling profile. Furthermore, both pits belong to the same period of Krania's settlement (between 9 th and 8 th centuries BC). However, the archaeological context might be distinct, the charcoal from the fillings of these two pits may therefore have different origins, i.e. construction wood or fuel for Pit A, or exclusively wood from several domestic fires for Pit B. The species diversity in the pit B (n=13) is as diverse as in the pit A (n=14) that does not support a possible selection for timber in Pit A. Pit A could have been subject to an accidental fire or sanitation and used in a second time as a detrital accumulation for example. As the charcoal are dispersed within both structures, they can correspond to hearth emptying (direct or indirect) and not necessarily correspond to the inner deposit layer of the pits. In both cases, no strict variability is observable from a structure to another, both pits have coherent assemblages, which supports the idea of the same exploited forest formation. Thus, the charcoal assemblage of Pit A and pit B can be put together to discuss about the firewood exploitation at Krania during the Iron Age.

Discussion

Environment and evidence for woodland management practices around Krania for the

Iron Age period

The charcoal assemblage from Krania consists of 14 different wood taxa from specific forest types (mixed deciduous forest, woodlands and shrubs, grasslands, xeric shrublands etc.), allowing us to examine the use of trees and shrubs in and around the ancient settlement.

Ecology, habitat and distribution

The assemblage contains three main taxa, including pines (Pinus spp.), oaks (Quercus spp.), and maple (Acer sp.). The first two genus could be found in the vicinity of the settlement, located in the lowlands. At Krania, both deciduous and evergreen oaks are represented and can grow in the same milieu on different soil types and with different slope inclinations. In Greece, they prefer shallow to moderate deep calcareous soils. Nine oak species can occupy the region of Macedonia (Q. cerris, Q. frainetto, Q. itharburensis, Q. petraea, Q. pubescens, Q. robur, Q. trojana, Q. ilex, and Q. coccifera; [START_REF] Dimopoulos | Vascular plants of Greece: An annotated checklist. -Berlin: Botanic Garden and Botanical Museum Berlin-Dahlem[END_REF]. Deciduous taxa prefer low to higher altitudes forests but some oak types can also grow in shrubby macchie (Q. coccifera, Q. pubescens, Q. trojana) or as individual trees in fields (Q. pubescens; [START_REF] Dimopoulos | Vascular plants of Greece: An annotated checklist. -Berlin: Botanic Garden and Botanical Museum Berlin-Dahlem[END_REF][START_REF] Strid | Wild Flowers of Mount Olympus. -Kifissia: Goulandris Natural History Museum[END_REF]. Evergreen oak is mostly present as an undergrowth of coniferous woodland in the foothills of Mount Olympus as well as evergreen shrubs in macchie, occasionally forming pure stands.

Considering the local environment of Krania, it is not surprising to have both types represented in almost the same proportions. The presence of pine tree within the assemblage fits with the dry and warm environment of the settlement. Pinus nigra characterized the most dominant forest at Mt Olympus (between ca. 500 to 1500m.a.s.l.), sometimes found in association with A. campestre, as well as it can be found in woodlands and shrubs, often forming pure stands especially on dry ridges and slopes [START_REF] Polunin | Flowers of Greece and the Balkans, a Field Guide[END_REF][START_REF] Dimopoulos | Vascular plants of Greece: An annotated checklist. -Berlin: Botanic Garden and Botanical Museum Berlin-Dahlem[END_REF]. At low altitudes, a gradual transition from macchie to P. nigra woodland is observable [START_REF] Strid | Wild Flowers of Mount Olympus. -Kifissia: Goulandris Natural History Museum[END_REF][START_REF] Dimopoulos | Vascular plants of Greece: An annotated checklist. -Berlin: Botanic Garden and Botanical Museum Berlin-Dahlem[END_REF]. Pinus sylvestris presents the same habitat's characteristics but can also form pure or mixed stands with Fagus sylvatica, Picea abies and Betula pendula [START_REF] Dimopoulos | Vascular plants of Greece: An annotated checklist. -Berlin: Botanic Garden and Botanical Museum Berlin-Dahlem[END_REF]. Concerning maple, the genus is associated to species that develop at higher altitudes such as Field maple (A. campestre) and Montpellier maple (A. monspessulanum), which on average grow from a minimum of 200 m.a.s.l. Other species such as Greek maple (A. heldreichii) can even grow from 900m. a.s.l. [START_REF] Strid | Wild Flowers of Mount Olympus. -Kifissia: Goulandris Natural History Museum[END_REF][START_REF] Banaszczak | The maples of Europe -Part 1[END_REF][START_REF] Dimopoulos | Vascular plants of Greece: An annotated checklist. -Berlin: Botanic Garden and Botanical Museum Berlin-Dahlem[END_REF]). Regarding their current ecological distribution, Pinus and Acer may occur at a higher altitude than the settlement of Krania.

As secondary taxa, cornelian cherry, is well represented in the assemblage. However, its distribution is heterogeneous and has a higher concentration in Pit A, especially in the sample NK185. It generally grows in altitudes between 200 to 1000 m. on slopes and forest edges, in mixed deciduous forests with oak and hornbeam [START_REF] Strid | Wild Flowers of Mount Olympus. -Kifissia: Goulandris Natural History Museum[END_REF][START_REF] Dimopoulos | Vascular plants of Greece: An annotated checklist. -Berlin: Botanic Garden and Botanical Museum Berlin-Dahlem[END_REF][START_REF] Scheppelmann | Flora Graeca (Edition Kentavros)[END_REF].

The other taxa that represent a minority within the corpus follow the same tendencies in terms of altitude, with an average ecological distribution around 200-300 m.a.s.l.

However, some taxa correspond to species that are also present in lower altitude environments. Cercis sp., Arbutus unedo, Platanus orientalis, Rhamnus saxatilis or Ulmus minor usually grow in coastal plain environments. Krania's location and environs is characterized via the anthracological material by mostly low and mid-high altitude species. The anthracological data coincide with species that can grow at altitudes that correspond to the near environment of the settlement, mostly below 600m. a.s.l. The principal plant community is characterized by woodland and shrub type (average altitude from 400 up to 1500 m. with lowest at 200m and highest up to 2200m), as maple (Acer spp.), hornbeam (Carpinus spp.), Cornus spp., ashes (Fraxinus spp.), pine trees (Pinus spp.), oriental plane (Platanus orientalis), poplars (Populus spp.), Salix spp., oaks (Quercus spp.). Olea europaea can also be found in this habitat, especially in dry rocky slopes.

Mixed deciduous forest with oaks (Quercus spp.), oriental hornbeam (Carpinus orientalis), ashes (Fraxinus spp.), and elms (Ulmus spp.) characterized the second main habitat with minimum altitude of 100m. up to 2200 m. for certain species (Stride 1980[START_REF] Dimopoulos | Vascular plants of Greece: An annotated checklist. -Berlin: Botanic Garden and Botanical Museum Berlin-Dahlem[END_REF][START_REF] Scheppelmann | Flora Graeca (Edition Kentavros)[END_REF]. While they can be part of different syntaxonomy (see 2.1), these vegetation units are also favourable for pine trees and elms, forming a sub-association to oak [START_REF] Pantera | Distribution and phytogeographical analysis of Quercus ithaburensis ssp. macrolepis in Greece[END_REF]). On the other hand, the representation of some plant communities, such as macchie or phrygana, by typical wood taxa present in the samples (notably Acer campestre, Arbutus unedo, Olea europaea and Rhamnus lycioides) are specific to a minimum altitude of 200 m.a.s.l.

Resource accessibility and woodland management

The diversity of wood taxa within Krania assemblage demonstrates a firewood procurement area probably around the settlement as most of the taxa can grow below 600 m. a.s.l. However, the topography is particularly contrasted in the vicinity of the site. This supports potential difficulties to access to the wooden resources, highlighting the capabilities of the Krania community to adapt to their environment regardless of constraints in accessibility, since the ascending elevation is higher than the altitude of the settlement itself (fig. 5). This suggests that firewood was preferably collected in low and mid-altitude and wood-pasture landscapes. The variability of the species associated to the two pits is a good environmental indicator that can be discussed in the frame of opportunist wood collection, following a least-effort pattern (Shackleton and Prins 1992, [START_REF] Asouti | Reconstructing woodland vegetation and its exploitation by Past Societies, based on the Analysis and Interpretation of Archaeological wood charcoal macro-remains[END_REF]. The palaeoecological representativeness of the frequency spectra of taxa can be established based on several parameters such as a large number of woody species of all types, reproducible observations across the synchronous strata and structures of a given site and anthracological taxon frequency spectra with high ecological consistency [START_REF] Chabal | Forêts et sociétés en Languedoc (Néolithique final, Antiquité tardive), L'anthracologie, méthode et paléoécologie[END_REF][START_REF] Chabal | Reconstructing the heterogeneity of past woodlands in anthracology using the spatial distribution of charcoals in archaeological layers: Applied to the postglacial occupation of the Abeurador cave (Hérault) in the South of France[END_REF]. However, it is not excluded that a part of the charcoal fragments corresponds to selected taxa, for timber or other craft products, whose by-products could have been used, in a second phase, as firewood.

Woodland management practices that result in the formation of anthropogenic habitats range from the protection of tree stands from browsing, clearance of herbaceous ground flora via burning or grazing and the selective removal of shrubs, to silvicultural practices (Asouti and[START_REF] Asouti | Holocene semi-arid oak woodlands in the Irano-Anatolian region of Southwest Asia: Natural or anthropogenic?[END_REF]Kabucku 2021). While it is difficult to address these issues based solely on the diversity of anthracological assemblages, additional informations about farming activities and wood use can be achieved by pollen studies that documents the potential regional vegetation, including wood and herbaceous taxa.

For the area of Pieria, local and precise information is sparse, apart from an important profile deriving from the area of Variko Litochoro [START_REF] Athanasiadis | Zur postglazialen Vegetationsentwicklung von Litochoro Katarinis und Pertouli Trikalon (Griechenland)[END_REF]. It is situated some 15 kilometers north of Krania, at an altitude of 148 m. a.s.l. The Late Bronze Age -Geometric period (1600-900 BC) is characterized by an increase in identified Pinus, a strong decrease in Quercus and the gradual decline of Alnus and Poaceae. The period from 900-200 BC is the most relevant to the present study, and shows a decrease in Alnus and Pinus findings, while the presence of Salix is expanding, potentially as a consequence of the Iron Age climate depression, that might have had an impact on floodplain vegetation at Litochoro [START_REF] Athanasiadis | Zur postglazialen Vegetationsentwicklung von Litochoro Katarinis und Pertouli Trikalon (Griechenland)[END_REF]. This observation accompanies a decrease in the presence of Vitis and Poaceae, potentially caused by stronger inundation [START_REF] Athanasiadis | Zur postglazialen Vegetationsentwicklung von Litochoro Katarinis und Pertouli Trikalon (Griechenland)[END_REF]). Later on, there is an increase in the values of Castanea, Juglans, Cerealia, Humulus-Cannabis and Vitis, indicating a possible change in the use of the landscape with more open areas, as well as a consequence of the Iron Age climate depression [START_REF] Athanasiadis | Zur postglazialen Vegetationsentwicklung von Litochoro Katarinis und Pertouli Trikalon (Griechenland)[END_REF][START_REF] Athanasiadis | Zur postglazialen Vegetationsentwicklung des Voras-Gebirges (Almopia -Griechenland)[END_REF][START_REF] Athanasiadis | Zur postglazialen Vegetationsentwicklung des Paikon-Gebirges (Nord Griechenland)[END_REF][START_REF] Athanasiadis | Palynological study of Holocene sediments from Lake Doirani in northern Greece[END_REF]. Indeed, the presence of open vegetation profiles accompanies the vegetation dynamic of the Katerini plain. During the Iron Age period, areas of open vegetation due to deforestation and agriculture [START_REF] Athanasiadis | Zur postglazialen Vegetationsentwicklung von Litochoro Katarinis und Pertouli Trikalon (Griechenland)[END_REF][START_REF] Margaritis | Archaeobotanical investigations at the geometric site of Krania, Southern Pieria in Macedonia, Greece[END_REF] might have had an impact on the forest vegetation profile within Pieria's immediate surroundings.

Although palynology provides complementary insights on human/environment interactions, anthracology gives direct evidence of the presence and spread of woodland species, which often go undetected by pollen analysis. In our case, poor and/or irregular pollen dispersers like riparian taxa such as Salicaceae, Acer, Populus or Fraxinus are often undetected in palynology [START_REF] Tal | Comparative flowering ecology of Fraxinus excelsior, Acer platanoides, Acer pseudoplatanus and Tilia cordata in the canopy of Leipzig's floodplain forest[END_REF] but can be present within charcoal assemblages. For example, some species such as Fraxinus are representative of forest communities which are on the interface between forest and grassland environments [START_REF] Brust | Les communautés végétales des interfaces forêt-prairie et leur environnement face aux cas d'afforestation et de déforestation[END_REF], reinforcing evidence of open areas that were potentially adjacent to water courses and running water.

Plane (Platanus) easily grows in this habitat, which corresponds to the local environment of Krania that is from sea level to ca. 900 m. The previous observation supports the presence of alder (Alnus) and hornbeam (Carpinus), which grow next to freshwater habitats and wet grounds in woods, forming woodland and scrub. The association of riparian vegetation, open areas vegetation (meadow, macchie) and woodland and shrub suggest that the exploitation of wood conducted nearby the settlement location is plausible.

But what was the average distance that these populations could range from the settlement of Krania to exploit wooden ressources? Although the taxa distribution is characteristic of the local environment of the settlement, some species that are typical of plant formations can provide evidence for this. The development of macchie in the region starts from 200 m.a.s.l. Considering this parameter, the nearest point to Krania that corresponds with this height is at a distance of ca. 900m. from the settlement. In addition, while the identified taxa can grow in altitudes from 0 m. onwards, the main forest communities (woodland and scrub and mixed deciduous forests) develop at an average altitude of a minimum of 200 m.

Considering that the settlement evolved within a defined perimeter, it fosters the idea that forest exploitation can be associated partially with degraded forest zones, that is to say macchie, particularly in low altitudes. The process of deforestation at low altitudes is supported by pollen records, but also by diatomea analyses for the Late Holocene within the region [START_REF] Lawson | Lateglacial and Holocene vegetation history at Nisi Fen and the Boras mountains, northern Greece[END_REF][START_REF] Kotthoff | Late glacial and Holocene vegetation dynamics in the Aegean region: an integrated view based on pollen data from marine and terrestrial archives[END_REF][START_REF] Zhang | Lateglacial and Holocene climate and environmental change in the northeastern Mediterranean region: diatom evidence from Lake Dojran (Republic of Macedonia/Greece)[END_REF]. A decline of temperate forests in ca. 3,500 cal yr BP and the reduction of the coniferous forest in ca.

2,500 cal yr BP is noticeable [START_REF] Wagner | A 40.000-year record of environmental change from ancient Lake Ohrid (Albania and Macedonia)[END_REF][START_REF] Zhang | Lateglacial and Holocene climate and environmental change in the northeastern Mediterranean region: diatom evidence from Lake Dojran (Republic of Macedonia/Greece)[END_REF]. Thus, human impact might have constituted an important factor to the nutrient erosion of the soil and its related deforestation via the expansion of agriculture and irrigation [START_REF] Zhang | Lateglacial and Holocene climate and environmental change in the northeastern Mediterranean region: diatom evidence from Lake Dojran (Republic of Macedonia/Greece)[END_REF]. Both anthropic and natural erosion may have had an impact on species distribution within the ecosystem.

The presence of rivers close to the settlement, however, might have contributed to the sufficient irrigation of potential fields, which is supported by the increase of cultivated taxa within the pollen record (Anathasiadis 1975) and the high concentration of grains [START_REF] Margaritis | Archaeobotanical investigations at the geometric site of Krania, Southern Pieria in Macedonia, Greece[END_REF].

In Krania, some of the secondary taxa include fruit trees such as Cornus, cornelian cherry (Cornus mas), and olive (Olea europaea). At low altitudes, rocky environments with thin soils on slopes present particularly favourable conditions for olive trees, benefiting their cultivation. This species can adapt to Xeric Mediterranean phrygana and grasslands, showing a perfect adaptability to Krania's environment. The presence of olive in the assemblage is not surprising as evidence of oleiculture is attested for the Iron Age in Northern Greece [START_REF] Foxhall | Olive Cultivation in Ancient Greece: Seeking the Ancient Economy[END_REF][START_REF] Pagnoux | Émergence, développement et diversification de l'arboriculture en Grèce du Néolithique à l'époque romaine : confrontation des données archéobotaniques, morphométriques, épigraphiques et littéraires[END_REF][START_REF] Valamoti | Did Greek colonization bring olive growing to the north? An integrated archaeobotanical investigation of the spread of Olea europaea in Greece from the 7 th to the 1 st millennium BC[END_REF][START_REF] Pratt | Oil, Wine, and the Cultural Economy of Ancient Greece: From the Bronze Age to the Archaic Era[END_REF], and in the assemblage from Krania [START_REF] Margaritis | Archaeobotanical investigations at the geometric site of Krania, Southern Pieria in Macedonia, Greece[END_REF]. While Pieria is situated in the foothills of Mount Olympus, we can hypothesize that the destruction of oak forests in deep soils by converting them to agricultural fields or cultivated terraces in the nearby environment of the settlement, might have offered to economic species such as the olive tree the opportunity to be more intensively cultivated at an average altitude of up to 300 m. This situation corresponds approximately to a distance of 1.5 kilometers from Krania.

Obviously we have to keep in mind that the actual soil profile is different from the ancient one, and the actual distribution of the vegetation within the landscape was relatively different. In addition, we have to mitigate the importance of olive within the assemblage.

The presence of olive stones in addition to the charcoal could give an indication of olive cultivation but few Olea charcoal is present in the pits, which could correspond, as for the rest of the corpus, to fuel strategy rather than pruning residues. Considering this observation, wood of wild olive tree could have been collected using the same nonselective strategy as was used for pines and oaks. This observation is reinforced by the presence of the fruit remains from Krania. Macroremains of cornelian cherry, pomegranate (Punica granatum) and grapevine (Vitis vinifera) are also identified at Krania (Margaritis 2007, Godefroy and Margaritis in preparation) and could be the result of gathering and tree cultivation. However, charcoal occurence of these three species does not have a homogeneous distribution either concentration in the charcoal assemblage of the two pits. From that perspective, Olea might have been used as fuel. However, those species belong to the fruit growing tradition of the Iron Age, particularly in open areas in phrygana and grasslands [START_REF] Pagnoux | Émergence, développement et diversification de l'arboriculture en Grèce du Néolithique à l'époque romaine : confrontation des données archéobotaniques, morphométriques, épigraphiques et littéraires[END_REF][START_REF] Foxhall | Households, Hierarchies, Territories and Landscapes in Bronze Age and Iron Age Greece[END_REF]. This raises the question, how were areas close to the site organized and utilized? Potential cleaning of woodlands and forests supports the idea of open areas within close vicinity of the settlement. This is also supported by the large amount of cereals, indicated from the macroremains [START_REF] Margaritis | Archaeobotanical investigations at the geometric site of Krania, Southern Pieria in Macedonia, Greece[END_REF].

Comparison of charcoal analyses from Iron Age Greece

As mentioned before, the few analyses of charcoal available in Greece make it difficult to draw comparisons for the Iron Age. The charcoal assemblages that are discussed here have been retrieved from only three archaeological sites, namely Corinth and Plakari in southern Euboia as well as Delphi in Phokis.

At Corinth, a single piece of charcoal belonging to a pine tree was identified [START_REF] Bookidis | Dining in the sanctuary of Demeter and Kore at Corinth[END_REF]. At Plakari, about 38 pieces of charcoal were identified from 3 ritual contexts; the assemblage is divided between the genus Quercus spp., Quercus cf. cerris and indeterminate taxa [START_REF] Lentjes | A walk in the woods. Charcoal analysis from three Greek archaeological sites: Geraki, Plakari and Titane[END_REF]. The concentration of oak may be connected to the ritual context, because of the specific selection of wood. At Krania, oak is also dominant but associated to an important taxonomical diversity, which reinforces the hypothesis of a non-selective strategy, especially considering the detrital context of pit A. However, the presence of deciduous oak at Plakari is interesting. Deciduous oak is linked to closed canopy forests while the evergreen type is specific to open environments like macchie.

Finally, the anthracological assemblage from the 9 th and 8 th centuries BC which belongs to the Chariot of the Rhodians at Delphi is so far the richest corpus for the period (Renault-Miskovsky and Thiébault 1997). Indeed, 282 charcoal were studied for the 9 th century BC while 1614 were analyzed for the 8 th century BC. For both periods a majority of Quercus ilex/coccifera is noticeable which may be due, as for Plakari, to the ritual profile of the context. A significant amount of coniferous wood, including Abies sp., cf. Cupressus, cf.

Juniperus and Pinus sp., is also of interest when thinking of the smoke and scent they can produce, especially for ritual purposes. As for Krania, the represented forest types belong mostly to mixed deciduous forest and woodland and shrubs, with additional presence of Phillyrea/Rhamnus, Phillyrea sp., cf. Populus and cf. Fraxinus. Legumes and fruit trees are also characteristic especially for the 8 th century B.C (Olea, cf. Ficus, Pomoideae, Prunus cf. domestica, Rosaceae, Anacardiaceae). The ritual activities that might have occurred in the settlement are noticeable when looking at the wood charcoal identified, as well as they bring evidences of the forest communities present in the surrounding of the settlement and anthropogenic landscapes (Renault-Miskovsky and Thiébault 1997).

Concluding remarks

The charcoal assemblage from Krania adds considerably to our understanding of the composition and exploitation of the woody environment surrounding the settlement.

Generally speaking, the assemblage presents a picture of a low and mid altitude woodland, with oaks, pine trees, and maple as well as alder, hornbeam, dogwood, ash, plane tree, poplar/willow, elm, and Phillyrea/buckthorn species. The forest community is primarily characterized by dry resistant and thermophilous taxa (Pinus type), evergreen and deciduous broad-leaved (Quercus spp., Acer sp. Olea sp., Cercis spp., Arbutus unedo, Ulmus sp., Phillyrea/Rhamnus), mainly found on the hilly lowlands and coastal areas [START_REF] Spanos | Forests of Greece, Their Multiple Functions and Uses, Sustainable Management and Biodiversity Conservation in the Face of Climate Change[END_REF]. The surrounding environment of the settlement can be pictured with partly forested areas, bushlands and grazing lands with partial erosion on the slopes due to anthropic and natural impacts. Riparian taxa accompany the presence of running water in the nearby environment of the settlement. Exploitation of wood might have been undertaken within walking distance of the settlement, particularly on the hilly slopes (average altitude of 200 m) where different forest communities coexisted. The combination of running water and fruit tree species reinforces the idea of local cultivation, which is also supported by the pollen records and the increase of crop production (Margaritis 2007, Godefroy and Margaritis in preparation). Examining charcoal from Krania demonstrates the capability of these ancient communities to adapt to their environment by maintaining their lands while also exploiting wood resources. The systematic study of charcoal provides a new element comprehending past communities, encouraging for its use and further development within archaeological research.

Figure captions/Table captions

Fig. 1 .

 1 Fig. 1. Location of the Krania settlement in Southern Pieria, Macedonia, Greece (map by J. Fokkema).

Fig. 2 .

 2 Fig. 2. A: Position of the Krania settlement at the foot of Platamon Hill (CAD by L. Godefroy). B: Excavation drawing that shows the different buildings, pebble floors and pits (E. Poulaki-Pantermali 2013).

Fig. 3 .

 3 Fig. 3. A: Picture of the oval building from the 8 th century B.C. (E. Poulaki-Pantermali 2013). B: Artist's impression of a single-room building from Krania (E. Poulaki-Pantermali 2013).

Fig. 4 .

 4 Fig. 4. SEM macrophotographs of anatomical sections of Iron Age wood from Krania (photos by L. Godefroy). A-C: transverse, tangential and radial sections of pine tree (Pinus cf. sylvestris/nigra). D-F: transverse section of Quercus sp. deciduous, transverse section of Quercus sp. evergreen, tangential section of oak (Quercus sp. deciduous). G-I: transverse, tangential and radial sections of judas tree (Cercis sp.). J-L: Transverse, tangential and radial section of cornelian cherry (Cornus mas). M-O : Transverse, tangential and radial section of olive tree (Olea europaea).

Fig. 5 .

 5 Fig. 5. Theoretical transect of the vegetation in the surrounding of the settlement of Krania (transect by L. Godefroy).

  . Various halophyte and psammophyte species can be found near the sea and stands of Platanus orientalis, Salix sp. and Populus alba characterized the foreshores. Quercus coccifera, Q. Quercion ilicis can bes associatiated with Quercus ilex, Q. coccifera, Pinus brutia, P. halepensis (Anathasiadis 1975, Dimopoulos et al. 2013). Dense scrub forests are composed of Crataegus monogyna, Rosa canina and Vitex agnus-castus (Anathasiadis 1975). Olea europaea var. oleaster, Fraxinus oxycarpa and Alnus glutinosa are notably found around the swampy areas near Litochoro (Anathasiadis 1975). At higher altitude, up to 400 to 700 m. a.s.l., mixed deciduous forests with notably ostryo-carpinion associations

	ilex, Q. pubescens, Q. trojana, Cistus, Juniperus oxycedrus, Laurus, Cercis siliquastrum, Acer campestre, A. monspessulanum, A. sempervirens, Rhamnus alaternus, Carpinus orientalis, Fraxinus angustifolia, F. ornus, Cornus, Pistacia, Arbutus unedo, A. andrachne, etc. are representative of macchie environment, characterized with hard leaf evergreen shrubs in near-coastal areas. Phrygana is also represented with Quercus ilex, Cistus, Rhamnus, Olea and Pistacia. Transitional and central European plant communities are found in association with Mediterranean sclerophyllous forests at an average altitude woods are replaced by Platanus orientalis, Alnus glutinosa, Populus alba, P. nigra, Salix alba, S. amplexicaulis, S. eleagnos, S. fragilis, S. rubens, S. triandra, Rhamnus alaternus etc. At higher altitude, other associations are present, such as Buxetum sempervirentis involving Quercus coccifera, Q. pubescens, Q. conferta, Carpinus orientalis, Pistacia terebinthus, Cotinus coggygria, Fraxinus ornus, Colutea arborescens, Acer monspessulanum, Ostrya carpinifolia, Salvia argentea, Campanula sprunerana, Parietaria officinalis, etc. Forming the high mountain vegetation, Quercetalia pubsecentis and Quercion conferta are present, with Q. pubescens, Q. conferta, Q. petraea, Acer campestre, A. hyrcanum, Juniperus oxycedrus, Corylus avellana, Sorbus torminalis, Ostrya carpinifolia, Carpinus betulus, Rhamnus fallax, that form notably small bushes. Other associations, such as Fagetalia forests, Pinion heldreichii, Abieton cephalonicae, of 200-300 m. are represented. Throughout river valleys and flood plains from the coastal hills evergreen Fagion moesiacae are distributed in the surrounding of Mount Olympus (Anathasiadis

Table 1 .

 1 Identification table of charcoal identified at Krania per samples (analyses by L. Godefroy).

		Sector	IV	IV	IV	IV	IV	IV	IV	IV	IV	IV	IV			III
		Trench	12D	12D	12D	12D	12D	12D	12D	12D	12D	12D	12D			11C 1D
		Stratum	E6	E6	E6	E6				E6	E6	E6	3A/3C
	Site : Krania	US	NK58	NK74	NK87	NK90	NK93 X	NK96	NK103	NK124	NK135 X NK146 X	NK185			NK280
	Director : E. Poulaki-Pantermali Year : 2001	Volume (cm3) Period	2 bags Geometric Geometric Geometric Geometric Geometric Geometric Geometric Geometric Geometric Geometric Geometric 20L 24L 10L 32L			12-18L Geometric		Total
		Structure	Pit A	Pit A	Pit A	Pit A	Pit A	Pit A	Pit A	Pit A	Pit A	Pit A	Pit A	Pit A total	Pit A	Pit B	Pit B
															Relative		Relative
			n/frgm.	n/frgm.	n/frgm.	n/frgm.	n/frgm.	n/frgm.	n/frgm.	n/frgm.	n/frgm.	n/frgm.	n/frgm.	n/frgm.	frequenc	n/frgm.	frequenc
															y		y
	Acer sp.	Maple	1	1	1		1	3	1	5	7			20	10%	15	21%	35
	cf. Acer	cf. Maple												0	0%		3%	2
	Alnus sp.	Alder								1				1	1%		0%	1
	Arbutus unedo	Strawberry tree					1			1				2	1%		4%	5
	Carpinus sp.	Hornbeam	1											1	1%		0%	1
	Cercis sp.	Judas tree, Redbud								1	1			2	1%		3%	4
	Cornus sp.	Dogwood											4	4	2%		1%	5
	Cornus mas	Cornelian cherry											21	21	11%		8%	27
	Fraxinus sp.	Ash	9		1							1	1	12	6%		1%	13
	Olea europaea	Olive tree				1			2			1	10	14	7%		4%	17
	cf. Olea europaea	cf. Olive tree											1	1	1%		3%	3
	Pinus cf. sylvestris/nigra	cf. Baltic pine/Black pine	1	5	7		4	12	1	4	1			35	18%		13%	44
	cf. Pinus	cf. Pine								2	1			3	2%		1%	4
	Pinus cf. peuce	cf. Macedonian pine	1			4	1			4				10	5%		0%	10
	Platanus sp.	Plane tree	2		1				1	1	2			7	4%		0%	7
	Populus / Salix	Poplar / Willows		2										2	1%		0%	2
	Quercus sp.	Oak			1					4				5	3%		6%	9
	cf. Quercus Quercus sp. deciduous Quercus sp. evergreen Phillyrea/Rhamnus cf. Phillyrea/Rhamnus cf. Salix	cf. Oak Oak sp. deciduous Oak sp. evergreen Phillyrea /Buckthorn cf. Phillyrea /Buckthorn cf. Willows	1	2	2 5 4 1		2 3	3 2	5 4	8 4	1		2 1 1	0 24 20 5 1 1	0% 12% 10% 3% 1% 1%		1% 10% 6% 1% 0% 0%	1 31 24 6 1 1
	Ulmus sp.	Elm												0	0%		3%	2
	Oleaceae									1				1	1%		1%	2
	Moraceae													0	0%		1%	1
	Maloideae													0	0%		1%	1
	Indeterminata	Undeterminated					2	1		2				5	3%		7%	10
	Total		16	10	23	5	14	21	14	38	13	2	41	197	100%	72	100%	269

Acknowledgements

This study was undertaken as part of the PhD studies of one of the authors (Lou Godefroy) in the frame of the project 'What went into the melting pot? Land-use, agriculture, and craft production as indicators for the contributions of Greek migrants and local inhabitants to the so-called Greek 'colonization' in Italy (ca. 800-550 BC)' that was made possible by a grant awarded by the Netherlands Research Council (NWO). We are grateful for the support received by the Vrije Universiteit of Amsterdam (VU), the National Museum of Natural History of Paris (MNHN), and the Cyprus Institute (STARC). Many thanks to Jan Paul Crielaard for his review, Jaap Fokkema (VU) and Maël Doré (MNHN) for their help with some of the figures, Efi Poulaki-Pantermali for providing the material from Krania for this study.

Data availability

Raw data are accessible in table 1.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.


Annie Simon
email: simon@icmcb-bordeaux.cnrs.fr

Jean Ravez

C N ] Li

Na + Mg

Ni 2+

Cu 2+

… Li

C N ] Mg

V 2+

Co 2+ Fe 2+

Zn 2+

V 3+

Mn 3+

Fe 3+

Ti 4+

Zr 4+

Sn 4+

Nb 4+

V 5+

Nb 5+

Ta 5+

W 5+

Mo 6+

Solid-state chemistry and non-linear properties of tetragonal tungsten bronzes materials

Keywords:

Non-linear properties of tetragonal tungsten bronze materials are related to solid-state chemistry. This family is very prolific, with its five different cationic or anionic crystallographic sites. The physical characteristics are correlated to composition and chemical bonding. Some effective or potential applications are described.

Introduction

Tetragonal tungsten bronze (TTB) structure was determined in 1949 by Magneli [1] on K 0.57 WO 3 (K 2.85 W 5 O 15 ). In 1953, ferroelectric properties were evidenced on PbNb 2 O 6 (Pb 2.5 Nb 5 O 15 ) [2]. In 1967, non-linear optics properties were studied on TTB crystals in IBM and Bell Telephone Laboratories [3]. From 1970, dielectric studies were carried out simultaneously in the USA, the USSR, Japan, Great Britain and France.

The present work refers to solid-state chemistry and non-linear properties (ferroelectricity, ferroelasticity, ferromagnetism, non-linear optics…) in TTB type materials. These properties are so called due to the main physical characteristics that display a non-linear variation (hysteresis loop) with external stress: polarizationelectric field, strain-mechanical stress, magnetizationmagnetic field… Relaxors will be also described. Materials involved are either ceramics or single crystals. Works on thin films are topical subject up to date.

Crystal chemistry

Ion localization

Table 1 gives the localization of various ions (or chemical entities) in the different crystallographic sites. The chemical stability is the highest when:

• big cations (e.g., Ba 2+ or Pb 2+ ) occupy an A site;

• small and low-charged cations (e.g. a Li + ) are in C site; • small and highly charged cations (e.g., Nb 5+ , Ta 5+ ) are in a M site; • for high-oxygen-rate compositions (e.g., O 15 ); • for high-fluorine-rate compositions (e.g., F 15 ).

The various types of stoichiometry depend on the existence of vacancies (□) in either one or another site: Several solid solutions can be obtained between each kind of non-stoichiometric compositions (e.g. (Ba 1. 5 Sr 0.5 )(Sr 0.5 □ 0.5 )□ 2 Nb 5 O 15 ).

• all sites full: A 2 BC 2

Phase transitions

In TTB crystals, there is a permanent anisotropy, even in centrosymmetric phases; it is not thus possible to reach the cubic symmetry. The prototype high temperature variety is tetragonal (4/mmm). The following Table 2 gives the various phase transition sequences with the corresponding properties as described in sections 3 and 5 [4]:

• no transition: it is the case of oxyfluorides with a relatively high fluorine substitution rate; very low degree of covalence in octahedral bonds prevent the formation of non-centro-symmetric phase; • one transition: when the temperature decreases, a ferroelectric phase appears in either the tetragonal system (4mm) or the orthorhombic one (mm2) giving rise here to ferroelastic properties. Polar axis is along [001] for spherical cations (e.g., alkaline and alkaline earth cations); it is oriented along [010] when cations with 6s 2 lone pair, easily polarizable, are introduced (Tl + , Pb 2+ , Bi 3+ ); • two transitions: compounds with tantalum belong to this family (Table 2); • three transitions were announced for Ba 2 NaNb 5 O 15 : a ferroelastic phase is included between two other more symmetric phases with the same point space group [5]. Such a transition sequence is in fact more complicated with appearance of incommensurate phases. From room temperature to 523 K, the first incommensurate phase is nearly commensurate. On the contrary, from 523 to 561 K, the corresponding phase is clearly incommensurate. These anomalous effects were attributed to the influence of defects having their origin in either an off-stoichiometry or off-equilibrium distribution of the cations in the structure [6].

Ferroelectricity

Generalities

Dielectric and pyroelectric studies have shown several TTB non-centrosymmetric phases to be ferroelectric. The values of T c of the various samples are between 7 K (K 3 Li 2 Ta 5 O 15 ) and 899 K (Ba 2.02 Li 0.96 Nb 5 O 15 ). Fig. 2 gives, as an example, the temperature dependence of the permittivity ε′ r for a ceramic with Pb 2 KNb 5 O 15 composition.

Origin of the ferroelectricity

Ferroelectrics may be divided into two basic types ('displacive' and 'order-disorder'):

• Displacive ferroelectrics: in the paraelectric phase, the small charged cation (e.g., Nb 5+ in Ba 2 KNb 5 O 15 ) is situated at the barycentre of the oxygen octahedron. The polar variety is characterized by correlated shifts of the Nb 5+ ions away from the centre of gravity of the octahedra. • Order-disorder ferroelectrics: while the ions in the paraelectric phase are located randomly in equiva- lent sites within the polar phase, they are preferentially positioned in some of these sites, thus giving rise to local polarization and thus ferroelectricity. The fluoride K 3 Fe 5 F 15 of mixed valency is an interesting example: in this case, it is the Fe 2+ and Fe 3+ ions that are arranged in non-equivalent octahedral sites [K 3 Fe 2+ (Fe 2+ 2 Fe 3+ 2 )F 15 ] by shifts along the polar axis.

Relation between composition, Curie temperature and chemical bonding

For a given structural type, the value of T c depends on the composition. The variations of T c may be correlated with factors of chemical bonding such as size, coordination or electronic configuration of cations, bond covalency, order-disorder, etc. [7,[START_REF] Ravez | Solid-State Inorg[END_REF].

Size of cations in the 6-coordination site

The value of T c decreases as the size of the M n+ ion increases. For the substitution Zr-Ti, for example, it is the largest size of Zr 4+ that limits its shift Δz, thereby leading to a decrease in T c [9] (e.g., T c = 518 K (Ba 3 TiNb 4 O 15 ); T c = 310 K (Ba 3 ZrNb 5 O 15 )).

Size of cations outside of octahedra

A decrease in the size of ion A n+ leads to a decrease in T c as long as its environment remains spherical and its coordination is not modified. Here are some characteristic examples (r Ba 2+ > r Sr 2+ ) [9]:

Compounds T c (K)

Bond covalency

The following three examples show this influence in displacive type ferroelectrics:

• the substitution Ta-Nb in the octahedral site leads to a large decrease in T c . The cations Nb 5+ and Ta 5+ have similar charges, configurations and size [9]. this is the effect of the covalence of the M-O bonds (M = Nb, Ta). In relatively covalent compositions such as the niobates, the increase in covalence (substitution Ta-Nb) arising from the more diffuse character of the wave function of Ta, i.e. a larger M-O overlap for M = Ta, leads to a drop in T c owing to the hardening or rigidification of the crystalline network, which thus cancels the ferroelectric distortion [10];

• the substitution F-O rapidly makes the network more ionic, since it leads to a decreased octahedral distortion. In most iono-covalent networks, this induces a spectacular drop in T c (Table 3) [11];

•

Fluorides

As seen in section 3.2., such ferroelectrics are of order-disorder type. The values of T c are here relatively high (T c = 495 K for K 3 Fe 5 F 15 ) [13].

Particular configurations of cations

The lone pair occurs for the Tl + , Pb 2+ and Bi 3+ cations. The increase in T c is due to this lone pair, which leads to a non-spherical cation. An example concerns K 2 BiNb 5 O 15 (T c = 633 K), which has a higher T c value than those of K 2 LaNb 5 O 15 (T c = 180 K) or K 2 NdNb 5 O 15 (T c = 442 K) [9].

Cationic order

An order within the crystalline network favours a cooperative effect, which in turn highly distorts the structure and induces an increase in T c . This effect may be demonstrated with the four following compounds: Sr 2 KNb 5 O 15 (T c = 429 K), Sr 2 NaNb 5 O 15 (T c = 539 K), Ba 2 KNb 5 O 15 (T c = 665 K), Ba 2 NaNb 5 O 15 (T c = 853 K). This distortion is not really enhanced by the disordered distribution of the alkaline earth cations and potassium in the sites of coordination 15 and 12, owing to their comparable size [9]. The polar phases remain tetragonal like the paraelectric phase. On the other hand, the smaller Na + ion is preferentially positioned in the site of coordination 12; the alkaline earth cations, which are twice as numerous, are situated naturally in the sites of coordination 15, themselves also twice as numerous.

Prediction of ferroelectricity

The transition from paraelectric to ferroelectric phase can occur through atomic shifts associated with distortion and/or rotation of octahedra. In both cases, when there is a transition from the non-polar to the polar form, low-amplitude atomic shifts Δz occur (< 1 Å) in the direction of the polar axis, thus giving a resultant polarization along the same axis. An empirical relationship between T c and Δz has been demonstrated: T c = 2 × 10 4 (Δz) 2 (Å 2 ) [14].

Examination of the atomic positions of a polar crystalline network (having no centre of symmetry but a polar axis) makes it possible not only to predict the ferroelectric properties but also the value corresponding of T c . Such relation was applied for various TTB crystals derived from (Ba,Sr) 2.5 Nb 5 O 15 , K 3 Li 2 Nb 5 O 15 …

Relaxors

Introduction

According to their particular dielectric behaviour, materials may be divided into two different classes, depending on whether they are classical ferroelectric or relaxor:

• a maximum of ε′ r and a value of the Curie temperature T c , no frequency-dependent for ferroelectrics; • a maximum of ε′ r at a temperature called T m , the two corresponding values varying with frequency for relaxors [15].

Concerning the relaxors, at least two different cations have to be localised in the same crystallographic site. Although relaxors have no macroscopic polarization (thus no P-E hysteresis loop), they are of great interest for dielectrics in capacitors and actuators [16]. The relaxor materials usually used are perovskite leadbased ceramics, for example PbMg 1/3 Nb 2/3 O 3 (PMN) and derived compounds. To date, various physical models have been described to explain the properties of relaxors [17]: compositional fluctuations and diffuse phase transition [18,19], the superparaelectric model [20], the nanostructure-octahedral model [21][22][23][24], the dipole-glass or the more recent derived model [25], the random-field model [26], the domain-wall model [27] and the random-layer model [28].

Lead-based TTB relaxors

Contrary to the perovskite class, the TTB Pb-based relaxors are not very numerous and have not been widely studied. However, some recent studies have been dedicated to this material class [17,29]. As an example, Fig. 3 shows the temperature dependence of ε′ r at various frequencies. At least one cation ferroelectrically active in the octahedral site must be present. The relaxor effect is simultaneously due to a disorder of the lead position and to a disorder of the cationic distribution in the A and B sites. The substitutions in the M site induce a relaxor state if the long-range order is not induced by a local dipolar order (e.g., tantalates).

Lead-free TTB relaxors

Such materials are of great interest as environmentfriendly applications. The first compositions announced as relaxors concerned the Ba 2.5(1-x) Sr 2.5x Nb 5 O 15 solid solution [30][31][32]. Very recently, relaxor behaviour was demonstrated in other compositions: e.g. BaLaNb 5 O 15 , BaBiNb 5 O 15 , SrK 2 Nb 5 O 14 F, BaNa 2 Nb 5 O 14 F… [33]. Some solid solutions situated between classical ferroelectric and relaxor have also been studied. Such is the case with A 2 B(Nb 1-x Ta x ) 5 O 15 : the replacement of very ferroelectrically active Nb 5+ by the less active Ta 5+ transforms the macroscopic polarization into a local one for the highest values of x. This is also the case with the Ba 2 NaNb 5 O 15 -BaNa 2 Nb 5 O 14 F system; for compositions close to the oxide, the behaviour is ferroelectric, while it becomes relaxor when the fluorine rate and thus the sodium rate is sufficiently high (Fig. 4) [34]. The frequency dependence of the permittivities of a ceramic with a composition BaNa 2 Nb 5 O 14 F clearly displays a frequency relaxation at 77 K, while it disappears at 300 K, i.e. for T > T m (T m = 140 K at 10 3 Hz) (Fig. 5). The crystal structure determination of this compound is in good agreement with the dielectric properties: the A-site is statistically occupied by equal quantities of Ba 2+ and Na + , thereby giving a cationic disorder in this A-site, while the B-site is filled by Na + cations [35].

Of all the lead-free compounds studied, it is with Bi 3+ , another 6 (sp) 2 lone pair cation, that the highest value of T m was obtained: T m (BaBiNb 5 O 15 = 319 K at 10 3 Hz). Such a result is in good agreement with that obtained for lead-free compositions derived from Ba-TiO 3 [36].

Ferroelasticity

The paraelectric-ferroelectric transition may be effected either without change of crystalline system (4/mmm → 4mm), or with the appearance of an orthorhombic symmetry due to slight distortions (4/mmm → mm2). In the last case, the ferroelectric phase has also ferroelastic properties.

When inserted cations in sites of 15-and 12-fold coordinations are alkaline or alkaline-earth ions (e.g., Ba 2 NaNb 5 O 15 ), the polar axis is along the [001] direction [37]. The switching of the polarization direction does not entail the passage from one ferroelastic state to another and inversely, the elastic transformation from state one to state two by the application of a mechanical stress does not change the sense of the polarization. Thus ferroelastic-ferroelectric properties are decoupled (Fig. 6).

On the contrary, if the inserted cation possesses a 6s 2 lone pair (Tl + , Pb 2+ , Bi 3+ ), the polar axis is along [010] and simultaneously the b parameter is higher than the a one [38,39]. We may foresee that the application of an electric field along Ox would produce a rotation of polarization by 90°and consequently the passage from an elastic state to another. In the same way, the change from state one to state two would lead to the rotation of the polar axis by 90°. The ferroelastic-ferroelectric properties would then be partially coupled. Such a prediction was proved on a lead potassium niobotantalate crystal [40].

Non-linear optics

The second harmonic generation efficiency E SHG is the rate of conversion of a fundamental wave into a second harmonic wave (100 I 2ω /I ω ). It is usually expressed as a relative value compared to that of a reference compound, e.g., KH 2 PO 4 (KDP), quartz α.

Ferroelectric materials (non-centrosymmetric and polar) have very high non-linear susceptibilities. The higher the spontaneous polarization P s , T c and Δz (see Section 3.4), the greater the tensor coefficients and the harmonic yield [41]. As an example, Fig. 7 shows the variation in E SHG /E SHG (KDP) versus T c [42]. Ba 2 NaNb 5 O 15 , Ba 2.14 Li 0.72 Nb 5 O 15 ) are among the most efficient compositions in order to be used in application field.

However, other factors are to be considered to obtain a maximal efficiency, i.e. crystal size and optical quality phase matching.

Ferromagnetism

Such a non-linear behaviour occurs in fluorides. As examples:

• in the (1-x) Mn 2+ -x Co 2+ substitution from K 2.5 (Mn 2.5 Fe 2.5 )F 15 , there is coexistence of antiferromagnetic and ferromagnetic short-range correlations above T N in the samples with a Co 2+ concentration in the range 0.6 < x < 1. Such a behaviour was also obtained for compositions containing Mn 2+ and Cr 3+ cations [43,44];

• in K 5x (M x 2+ M 1-x 3+
) 5 F 15 with 0.4 < x < 0.6, M 2+ = Mn, Ni and M 3+ = V, Cr, magnetic ordering occurs in two steps. Linear trimers (M 2+ ) 3 and (M 3+ ) 3 form exist, giving rise to a quasi-paramagnetic region, followed at lower temperature by long-range magnetic ordering. The 180°M 2+ -F --M 3+ magnetic interactions appear to be ferromagnetic [13];

• the composition K 3 Fe 5 F 15 is a multiferroic system.

Ferroelasticity and ferroelectricity are coupled at temperature lower than T c = 490 K. In addition, a spontaneous ferromagnetic component appears below 120 K, in the magnetically ordered phase [13,45,46].

Applications

Up-to-date TTB-type materials are now mainly used as single crystals, e.g.:

• strontium barium niobates as infrared detectors;

• barium sodium niobates as electro-optics modulators, parametric modulators or frequency doublers [47][48][49][50].

Elsewhere acoustic applications (e.g., sonars) require a very high k 33 electromechanical coupling coefficient. Unfortunately, big single crystals, useful for such an application, are often very difficult to elaborate. In this way, the current researches are oriented toward texturised ceramics. Such materials are prepared using small 'non-cubic' single crystals, obtained by flux methods and mixed with nanometric powder. Before sintering, it is necessary to line up the small single crystals in the same direction. TTB ferroelectric compositions possess a high value of k 33 , a tetragonal prototype phase and crystallise as long straight prism with a small square-base morphology. They are thus very promising candidates [51]. In addition, current researches are related to spread the application domain to TTB type thin films.

Conclusions

The TTB compositions are oxides, oxyfluorides or fluorides. The great ability of this prolific family to give various crystallographic distortions leads to performing ferroelastic and ferroelectric properties. Their physical characteristics (ferroelectric or ferroelastic Curie temperature T c , spontaneous strain or polarization) can be related to cationic size and charge, bond covalency and cationic order. An interesting result concerns the oxyfluorides ferroelectric (of displacive type) in which the value of T c decreases strongly with the fluorine substitution rate; however the pure fluorides (of ferroelectric order-disorder type) show unexpected high values of T c . The TTB materials are able to lead to a relaxor behaviour due to cationic disorder in one or another crystallographic site. The compositions with a high ferroelectric Curie temperature show a high second harmonic generation efficiency (non-linear optics). A ferromagnetic component was observed in fluorides containing transition element cations; the values of T N are nevertheless low. Some considerations of present or future applications were discussed. The non-cubic state of the prototypic phase is interesting for texturised ceramics in the way of acoustic applications. Also, lead-free compositions are of interest as environment-friendly applications.

2. 1 .

 1 Fig.1shows a schematic representation of the TTB structure projection along the [001] direction. For a general A 2 BC 2 M 5 X 15 (X = O, F) formulation, large ca-

  M 5 X 15 (e.g., K 3 Li 2 Nb 5 O 15 ); • half C sites empty: A 2 BC□M 5 X 15 (e.g., K 3 NiNb 5 O 15 ); • C sites empty: A 2 B□ 2 M 5 X 15 (e.g., Ba 2 NaNb 5 O 15 ); • B and C sites empty: A 2 □□ 2 M 5 X 15 (e.g., BaBiNb 5 O 15 ).

Fig. 1 .

 1 Fig. 1. Schematic projection of the TTB crystalline network along the fourfold c axis (A, B and C correspond to cationic sites with 15, 12 and 9 coordination numbers, respectively, in unit cell A 2 BC 2 M 5 X 15 ).

Fig. 2 .

 2 Fig. 2. Temperature dependence of the permittivity ε′ r for a ferroelectric ceramic: Pb 2 KNb 5 O 15 .

  an apparently unexpected case combines the two precedent phenomena. It concerns the F-O substitution in the very covalent network of some tantalates. For example, although in the niobate K 3 Li 2 Nb 5 O 15 (T c = 703 K), the F-O substitution leads to a very sharp drop in T c , in the tantalate K 3 Li 2 Ta 5 O 15 (T c = 7 K), T c increases (T c = 80 K) for K 3 Li 1.5 Ta 5 O 14.5 F 0.5 . In this case, it is the decrease in covalence due to the F-O substitution, which leads to a lesser hardening of the network, and thus to a slight increase in both the ferroelectric distortion and T c[12].

Fig. 3 .

 3 Fig. 3. Temperature and frequency dependences of the permittivity ε′ r for a relaxor ceramic Sr 2 K(Nb 0.775 Ta 0.225 )O 15 .

Fig. 4 .

 4 Fig. 4. Variation of transitions temperatures with x for ceramics with composition Ba 2-x Na 1+x Nb 5 O 15-x F x .

Fig. 5 .

 5 Fig. 5. Frequencies dependences of the permittivities ε′ r and ε′′ r for a ceramic with composition BaNa 2 Nb 5 O 14 F. The dashed line shows the fit of data according to the Cole-Cole model.

Fig. 6 .

 6 Fig. 6. Nature of spontaneous strains and polarizations in crystalline phases with TTB structure.

Fig. 7 .

 7 Fig. 7. Variation of SHG efficiency of TTB-type materials in relation to KDP as T c changes.

Table 1

 1 Localization of various ions or chemical groupements in the different crystallographic sites

Table 2

 2

	Transition sequences of phases with tetragonal tungsten bronze structure
	N a	Compositions			Transitions sequences
	0	K 3 Nb 5 O 13 F 2	4/mmm
			paraelectric
			paraelastic
	1	K 3 Li 2 Nb 5 O 15	4mm		4/mmm
			ferroelectric [001]	⇒	paraelectric
			paraelastic		paraelastic
		Sr 2 K 0.5 Li 0.5	mm2		4 / mmm
		Nb 5 O 15	ferroelectric [001]	⇒	paraelectric
			ferroelastic		paraelastic
		Pb 2 KNb 5 O 15	mm2		4 / mmm
			ferroelectric [010]	⇒	paraelectric
			ferroelastic		paraelastic
	2	S r 2 KTa 5 O 15	mm2		mmm		4/mmm
			ferroelectric [001]	⇒	paraelectric	⇒	paraelectric
			ferroelastic		ferroelastic		paraelastic
		Pb 2.07 K 0.56 Nb 0.91	mm2		mmm		4/mmm
		Ta 4.15 O 15	ferroelectric [010]	⇒	paraelectric	⇒	paraelectric
			ferroelastic		ferroelastic		paraelastic
	3	B a 2 NaNb 5 O 15	4mm		mm2		4 mm		4/mm
			ferroelec. [001]	⇒	ferroelec. [001]	⇒	ferroelec. [001]	⇒	paraelectric
			paraelastic		ferroelastic		paraelastic		paraelastic
		Ba 2.14 Li 0.71 Nb 2.5	222		mm2		42m		4/mmm
		Ta 2.5 O 15	antiferroelec.	⇒	ferroelec. [001]	⇒	paraelec.	⇒	paraelectric
			ferroelastic		ferroelastic		paraelastic		paraelastic
	a Transition number.

Table 3

 3 Variation of T c with the F-O substitution rate Compositions ΔT c (K) a (Sr 2-x K 1+x )Nb 5 O 15-x F x 140 (Ba 2-x Na 1+x )Nb 5 O 15-x F x 660 Sr 2 K(Nb 5-x Ti x )O 15-x F x 210 Ba 2 Na(Nb 5-x Ti x )O 15-x F x 115 a Normalized variation of T c for a ratio F/(F+O) varying from 0 to 10 -2 .