Variable temperature X-ray diffraction and high field EPR study of phase transition in multiferroic mixed ammonium peroxychromates, Rb1.36 (NH4)1.64Cr(O2)4

Abstract

Two cation sites in a unit cell with varying cation (NH4+) symmetry are rare, and Rb1.36(NH4)1.64Cr(O2)4 is one such compound with distinct cation sites. We report a systematic X-ray diffraction study using both single crystals and powder samples of Rb1.36(NH4)1.64Cr(O2)4, changes in the lattice parameters and symmetry as a function of temperature at a wide range (10–300 K). Pseudo octahedral NH4+ ions at cation sites, N2, are involved in NH⋯O hydrogen bond to yield a continuous second order phase transition in Rb1.36(NH4)1.64Cr(O2)4 in contrast to a first order step like irregular pattern in its parent compound (NH4)3Cr(O2)4. High field EPR studies of Cr5+ at a temperature range 10–300 K revealed a distortion in Cr(O2)4 groups in the plane (001) suggesting strong NH⋯O hydrogen bonding between (NH4) ions and Cr (O2)4 groups.