JOURNAL OF MATERIALS SCIENCE 33 (1998) 2479 — 2497 Review Materials belonging to the CrVO 4 structure type: preparation, crystal chemistry and physicochemical properties E. J. BARAN Centro de Quimica Inorga & nica (CEQUINOR), Facultad de Ciencias Exactas, Universidad Nacional de la Plata, C. Correo 962, 1990-La Plata, Argentina E-mail: baran@nahuel.biol.unlp.edu.ar Crystallographic data of vanadates, phosphates, chromates, sulphates and selenates belonging to the CrVO 4 (-CrPO 4 ) structure type and of some of its most important polymorphs, are reported. Characteristic structural peculiarities and the effect of pressure on the phase transformations are discussed. A definitive stability field for this structure type could be established. Synthetic procedures for the preparation of the different compounds adopting this and closely related structures are presented and its thermal, spectroscopic and magnetic properties are discussed. A brief overview on the most important characteristics of materials adopting the -CrPO 4 structure is also presented. 1998 Chapman & Hall 1. Introduction The structure of CrVO , described about 50 years ago, is one of the typical ABO structure types ad- opted by different oxide systems. Although the num- ber of compounds with this structure (often also called -CrPO or NiSO structure) is rather small, new materials belonging to this structure type have re- cently been prepared and characterized. Therefore, it seems timely to give a general, systematic overview of the crystal chemistry and of the most important phys- ical and chemical properties of these materials. To begin with, some general structural features are presented of the different materials adopting this structure, including comments and remarks on their most common and important polymorphs, followed by a general crystallochemical discussion. In the second part, some aspects related to the synthesis of these materials are presented and dis- cussed. The third part is devoted to the discussion of the most relevant physicochemical properties of these compounds, emphasizing thermal, spectroscopic and magnetic properties. The final section offers a brief overview of the few materials belonging to the -CrPO structural type. 2. Structural aspects 2.1. Main characteristics of the CrVO 4 structure CrVO is orthorhombic space group D —Cmcm— , with Z"4 [1]. The structure consists of infinite chains of trans edge-sharing CrO octahedra which run parallel to the c-axis. These chains are linked together by VO tetrahedra, as shown schematically in Fig. 1. The metal atoms are in the following special positions: Cr (4a) and V (4c). As adjacent elements in the periodic table, vanadium and chro- mium have nearly the same X-ray scattering powers and it was initially difficult to assess which atom is in position 4a and which in 4c. However, by means of X-ray absorption spectroscopy it became evident that vanadium was the tetrahedrally coordinated ion [2]. The CrVO structure has a fairly high tolerance and can accommodate octahedral ions as small as Fe(III) and as large as Tl(III) and Cd(II). Five classes of ABO materials are found in this structural type: trivalent cation phosphates and vana- dates, MPO ,MVO , and divalent cation sul- phates, selenates and chromates, MSO ,MSeO , MCrO . Only recently, LiMnO has been shown to be the first, and so far unique, permanganate adopting this structure. An analysis of the structural information available so far for ABO compounds shows that for phases with r (C.N."6) between 0.07 and 0.10 nm and r (C.N."4)(0.060 nm, CrVO is the only possible structure [3, 4]. With increasing A cationic radius, zircon, monazite or barite type structures are preferred; with increasing B radius the structural type changes substantially, generating octahedral coordination of this ion. A greatly simplified struc- tural field map, in the region around CrVO , is shown in Fig. 2. 0022—2461 1998 Chapman & Hall 2479