Paramagnetic resonance of the (Cr⁵⁺O₄)H₂ Halperin-Varma center in NH₄H₂AsO₄ and KH₂AsO₄

Abstract

The electron paramagnetic resonance (EPR) spectrum of Cr5+(3 d1, S=1/2) in undeuterated and 58%-deuterated NH4H2AsO4 was investigated. The EPR g-value tensors in the paraelectric and antiferroelectric phases are almost the same as those observed at high and low temperatures in KH2PO4 and KH2AsO4. This shows that the (Cr5+O4)H2 complex is the same in all crystals, i.e., a wave function of dx2-y2 character coupled to two lateral protons which reorient among the four possible Slater configurations in the paraelectric phase. The remarkable isotope shift of the local dynamic reorientational slowing-down observed in KH2PO4 and KH2AsO4 in approximate proportion to the shift of the bulk ferroelectric transition temperatures TcF, and the antiferroelectric TcAF of NH4H2AsO4, is analyzed qualitatively. It is shown to result mainly from the isotope effect of the short-range interaction Jsr via protons and deuterons for the impurity and for the bulk. Q-band data of the (Cr5+O4)H2 center in 75%-deuterated KH2AsO4 are also reported. An averaged high- and nonaveraged low-temperature EPR spectrum is observed in a temperature range of 250 to 290 K. The intensity ratio of the two follows an exp 2(T′-T)/Δ law over four orders of magnitude at T=266 to 273 K and Δ=5.3 to 6.1 K depending on the orientation of the external magnetic field. This novel result in motional narrowing is analyzed analytically to be compatible with a distribution PA of τ0(T′, T, Δ) of half width Δ, in reorientation times {Mathematical expression} with E=0.23 ±0.02 eV, {Mathematical expression}, probably resulting from the statistical occupation of deuterium atoms among the O--H-O bridges. Comparison with a theory of Binder definitely proves the extrinsic slowing-down and thus Halperin-Varma type character. In the range of temperatures investigated no local freeze-out has been detected. © 1978 Springer-Verlag.