EPR determination of three-dimensional correlations below the ferroelectric phase transition in pseudo-one-dimensional CsH2PO4:Cu2+

Abstract

The phase transition in monoclinic CsH2P04 (CDP) exhibits a number of unusual properties which are characteristic of one-dimensional systems. ' The ferroelectricity in CDP is associated with the ordering of hydrogen between two equilibrium sites in the short 0-H 0 bonds linking the P04 groups into zigzag chains parallel to the b axis. These chains are linked into two-dimensional (2D) layers by ordered hydrogen bonds lying roughly along the c axis and cesium atoms lying between these hydrogenbonded planes. s There are no hydrogen bonds along the a axis and the crystal has a cleavage plane perpendicular to the a axis. Neutron scattering studies show that significant short-range order develops along the chains at a temperature well above T,. The weaker interchain coupling responsible for the ferroelectric transition causes virtually no correlation between the layers until one gets quite close to T, . The dielectric constant in CDP shows a crossover between a region 3 K & T —T, ~90 K dominated by 1D correlations and a region 0 K & T —T, «3 K dominated by 30 correlations. The 10 correlation has been confirmed by s'P and deuteron resonance studies for partially deuterated CDP. 4 An apparent deviation from the quasione-dimensional model has been found, however, in the region of 0 K& T —T, ~20 K by Deguchi, Okau, and Nakamura. The aim of this work is to examine the temperature dependence of the order parameter of the phase transition by means of an EPR study of the line splitting for the fine-structure component of the Cu2+ tetramer spectrum in CDP. CsH2P04 doped with 0.3 mol% CuC12 2H20 shows the usual Cu + S —, ' single-ion spectrum at room temperature. Below about 250 K the fine-structure spectra were observed and described by us as arising from the Cu + tetramers with an effective spin S =2 and spin-Hamiltonian parameters: g ~~ =2.2575, g& =2.1866, D 0.181 cm ', E = —0.081 cm ', a =0.001 cm ', b 0, and c —0.021 crn '. Also the 0, —1 transition within the 5 =1 multiplet of the tetramer was reported to have approximate parameters D„0.543 and E„—0.159 cm obtained from a phenomenological theory of the tetrameric clusters. The superexchange interaction between pairs of Cu +(S 1) complexes arises through Cu-0-P-0-Cu paths. Both pairs are coupled by the hydrogen bonds along the c axis. The tetramer model is shown in Fig. 1. The excess charge compensation for the Cu + ion which replaces the Cs+ ion is presumably provided by a proton vacancy. Figure 2 shows the EPR line splitting for the 0 —1 transition for the S 1 multiplet versus temperature. The EPR line position above T, for the 0 —1 transition is given by the expressions