Effect of Uniaxial Stress upon the Successive Phase Transitions in K2PbCu(NO2)6

Abstract

The effect of uniaxial compressive stress upon the successive phase transition temperatures of K 2 PbCu(NO 2 )O 6 was investigated. The I-II transition temperature (280 K at 1 bar) increased linearly with the rate of 73 ±2 deg kbar 1 for the [001] stress, 2.5 ±0.2 deg kbar -1 for the [111] stress and 13.5 ±1.1 deg kbar -1 for the [110] stress, and the II-III transition temperature (273 K at 1 bar) increased linearly with the rate of 10.4 ±0.2 deg kbar -1 for the [001] stress, 5.8 ±0.5 deg kbar -1 for the [111] stress and 7.2 ±1.3 deg kbar -1 for the [110] stress, respectively. In the case of the compression along the [001] axis, the I-II transition which is of first order at the atmospheric pressure became of second order above around 0.4 kbar. The results are discussed on the basis of an orientational order-disorder model of the Jahn-Teller distorted Cu(NO 2 ) 6 octahedra by taking into account a coupling between the orientation of the octahedron and the bulk distortion induced by the pressure.