High-pressure behaviour of synthetic weberite-type Mn 2þ 2 Sb 5þ 2 O 7 : An in situ single-crystal X-ray study L. Chelazzi a , T. Boffa Ballaran b , G.O. Lepore c , L. Bindi c, d , P. Bonazzi c, * a Dipartimento di Chimica “G. Ciamician”, Università di Bologna, Via Selmi, 2, I-40100 Bologna, Italy b Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth D-95440, Germany c Dipartimento di Scienze della Terra, Università di Firenze, Via La Pira 4, I-50121 Firenze, Italy d C.N.R., Istituto di Geoscienze e Georisorse, Sezione di Firenze, Via La Pira 4, I-50121 Firenze, Italy article info Article history: Received 29 January 2013 Received in revised form 18 April 2013 Accepted 19 April 2013 Available online 3 May 2013 Keywords: Weberite Antimonates High pressure Single-crystal X-ray diffraction Compressibility Thermodynamic properties abstract An in situ high-pressure X-ray diffraction study has been carried out at room temperature up to 9.26 GPa on synthetic Mn 2 Sb 2 O 7 having a weberite-3T structure. A 2nd-order BircheMurnaghan Equation of State (EoS) was used to refine the pressureevolume data. The refinement of the unit-cell volume and of the isothermal bulk modulus at room pressure leads to: V 0 ¼ 782.7(2), K T0 ¼ 150(1) GPa. Unit-cell parameters decrease gradually as a function of pressure with a bulk modulus anisotropy scheme, with a being the softest direction. The overall mean polyhedral distances are quite constant, indicating a scarce compressibility of both the A and B polyhedra in the pressure range investigated. The compressional behaviour of Mn 2 Sb 2 O 7 is compared with that shown by ingersonite, Ca 3 MnSb 4 O 14 , and synthetic orthorhombic Ca 2 Sb 2 O 7 . Ó 2013 Elsevier Masson SAS. All rights reserved. 1. Introduction Among oxides and fluorides with general formula A 2 B 2 X 7 (where A is a medium-large cation and B is an octahedrally coor- dinated, high-charge cation) there are many compounds widely studied for their great technological potential owing to their ferroelectric and/or magnetic properties [1e22]. Most of them adopt a weberite-polytype structure or a zirconolite-polytype (commonly pyrochlore-like) depending on the kind of cations (ionic radius, electronegativity, etc.), as well as the pressure and/or temperature of crystallization. Using an anion-centred polyhedral description both structural types can be described as anion- deficient fluorite derivative with A and B cations forming a face- centred cubic array with the anions differently occupying the tetrahedral or octahedral interstices. In particular, in zirconolite- polytype structures, including the undistorted cubic pyrochlore structure as well as polytypes with lower symmetry [23e26], all anions occupy the tetrahedral cavities. On the other hand, the anion distribution is not maintained in weberite-type compounds, where one of the anions moves from tetrahedral interstice to an adjacent octahedral interstice. This arrangement, as well illustrated by Grey et al. [27], is a common feature of all weberite-type polytypes. The driving forces controlling a given compound to crystallize in one of these two competing structural type (i.e., weberite- or zirconolite-type) are not yet entirely identified. According to Cai and Nino [28], the A 2 B 2 O 7 compounds can be distinguished into zirconolite-type and weberite-type on the basis of the relative ionicity of the AeO bond and the R A /R B ratio, with weberite-type stabilized by relatively higher ionicity of the AeO bond and higher R A /R B ratios. However, the cations distinctiveness alone does not explain why a given compound can occur in the two competing structures and, in this regard, why pressure and temperature con- ditions adopted to synthesize the compound play a key role. Among the known M 2þ 2 Sb 2 O 7 oxides, the mineral ingersonite (M 2þ ¼ 3/4Ca þ 1/4Mn), which falls near the transition zone of the two competing structures in the plot of the relative ionicity of the AeO bond versus the R A /R B ratio proposed by Cai e Nino (Fig. 14b in Ref. [28]), crystallizes with the weberite-3T structure [29]. Never- theless, the pure compounds Ca 2 Sb 2 O 7 and Mn 2þ 2 Sb 2 O 7 , which fall relatively far from the transition zone of the two competing structures in the plot, occur in the two structural types depending on synthesis conditions. Ca 2 Sb 2 O 7 crystallizes as pyrochlore and * Corresponding author. Tel.: þ39 (0)55 2757532. E-mail address: paola.bonazzi@unifi.it (P. Bonazzi). Contents lists available at SciVerse ScienceDirect Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie 1293-2558/$ e see front matter Ó 2013 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.solidstatesciences.2013.04.012 Solid State Sciences 21 (2013) 85e90