Abstract
We determine the nature of coupled phonons in mixed crystal of Cs0.9(NH4)0.1H2AsO4 using inelastic light scattering studies in the temperature range of 5 K to 300 K covering a spectral range of 60–1100 cm−1. The phase transition in this system are marked by the splitting of phonon modes, appearance of new modes and anomalies in the frequency as well as linewidth of the phonon modes near transition temperature. In particular, we observed the splitting of symmetric (v1) and antisymmetric (v3) stretching vibrations associated with AsO4 tetrahedra below transition temperature ($T_c^*$Tc* ∼ 110 K) attributed to the lowering of site symmetry of AsO4 in orthorhombic phase below transition temperature. In addition, the step-up (hardening) and step-down (softening) of the AsO4 bending vibrations (v4 (S9, S11) and v2 (S6)) below transition temperature signals the rapid development of long range ferroelectric order and proton ordering. The lowest frequency phonon (S1) mode observed at ∼92 cm−1 shows anomalous blue shift (∼12 %) from 300 K to 5 K with no sharp transition near $T_c^*$Tc* unlike other observed phonon modes signaling its potential coupling with the proton tunneling mode.
Highlights
INTRODUCTIONAll the four modes associated with the internal vibrations of the AsO4 showed anomalous behaviour at ∼80 K attributed to the onset of the short range random potential fixing of protons
Mixed crystal of ferroelectric and anti-ferroelectric systems such as A1−xBxH2CO4 (A = K, Rb, Cs; B = NH4, ND4; and C = P, As) have attracted much attention in the recent times as they provide excellent medium to study the rich physics of competing interaction[1,2] such as protonglass formation, ferroelectricity and structural phase transition with temperature, as well as for their potential applications
We observed the splitting of symmetric (v1) and antisymmetric (v3) stretching vibrations associated with AsO4 tetrahedra below transition temperature (Tc∗ ∼ 110 K) attributed to the lowering of site symmetry of AsO4 in orthorhombic phase below transition temperature
Summary
All the four modes associated with the internal vibrations of the AsO4 showed anomalous behaviour at ∼80 K attributed to the onset of the short range random potential fixing of protons. Our detailed temperature-dependent Raman study shows different variations in the phonon modes frequency and linewidths. Our study reveals strong phonon renormalization below the transition temperature (Tc∗) from high temperature tetragonal to low temperature orthorhombic phase as well appearance of new phonon modes and splitting of symmetric and antisymmetric stretching vibrations associated with the AsO4 tetrahedra. We have observed sharp change of the frequency as well as linewidth of some of the modes associated with the arsenate ions near the phase transition temperature (Tc∗). The anomalous shift in the frequency near Tc∗ is attributed to the change of intermolecular interaction caused by proton ordering. The lowest phonon mode (S1) observed at ∼92 cm−1 doesn’t show the drastic change near the transition temperature unlike other modes, and it shows anomalous hardening (∼12 %) with decreasing temperature possibly due to its coupling with the proton tunneling mode
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