Ferroelectric and proton conducting behavior of a new elpasolite-related vanadium oxyfluoride (NH 4 ,K) 3 VO 2 F 4
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A new elpasolite-type (NH4,K)3VO2F4 compound was prepared and characterized by x-ray diffraction, differential scanning calorimeter (DSC), impedance analysis, and electrical polarization measurements. It crystallizes in an orthorhombic lattice with unit˚ , V ¼ 1041.04(11) A ˚ 3, cell parameters: a ¼ 8.9584(4), b ¼ 18.6910(14), c ¼ 6.2174(4) A þ þ Z ¼ 6. NH4 , and K ions are distributed statistically over crystallographically four equivalent sites. There are two distinguishable vanadium atoms forming cis- and trans-VO2F4 octahedra present in the unit cell. High-temperature studies by DSC and in situ x-ray diffraction revealed a first-order structural transformation from orthorhombic to cubic lattice around 343 K. Impedance measurements show two different kinds of conductivity behaviors for the two phases. In orthorhombic phase a significant conductivity resulting from involvement of protonic species is observed. In the orthorhombic phase, a clear ferroelectric hysteresis loop is observed.
I. INTRODUCTION
A2BMX6-type compounds belong to a large structural family, namely, elpasolite (K2NaAlF6) or cryolite (Na3AlF6),1,2 where BX6 and MX6 octahedral units form a three-dimensional network by sharing the corner X atoms. The cubo-octahedral cavities formed by cornersharing arrangements of the BX6 and MX6 octahedra are filled by the A ions. Similar to the perovskite lattice, the formation of the elpasolite structure is limited by the tolerance factor defined by the ionic radii of the constituent ions. The elpasolite-type compounds with halides and oxides as X ions have been most extensively studied.1,2 All these compounds have cubic (Fm3m) structure at certain temperature. In the cubic elpasolite structure, the B and M ions are occupied on 4b (½ 0 0) and 4a (0 0 0) sites, respectively, while A ions are located in 8c (¼ ¼ ¼) sites of the space group Fm3m. However, the location of the anions vary from the single (24e: x 0 0) to partially occupied multi sites.1,3 A highly disordered anion sublattice is formed due to the partially occupied anion sites. Most of these compounds crystallize in lower symmetric lattices as a result of bond stress or tilting of the octahedral units.1,4 In addition, several of these compounds exhibit successive phase transition under temperature or pressure because of (i) small shift of the X and/or A ions, (ii) distortion in the octahedral units, such as a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0175 J. Mater. Res., Vol. 25, No. 7, Jul 2010
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Jahn-Teller or off-center displacement of the B or M ions, and (iii) disordering of the cations or anions.1,2,4 The structural transformation in the elpasolite (Fm 3m)related compositions have been reported to have ferroelectric or ferroelastic nature or both.1,2,5,6 The elpasolite-type oxyhalides, in particular oxyfluorides have drawn significant attention because of their ferroelectric or ferroelastic properties due to lowering of the local or glo
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