Abstract
The librational lattice modulation in incommensurate phases of betaine calcium chloride dihydrate (BCCD) crystals was investigated by the EPR spectra of VO2+ impurities substituted for Ca2+ ions in the lattice. Below 164 K, the spectra exhibited anomalously broadened 51V-hyperfine lines, representing pinned condensates of librational order parameters. The angular variation of the anomalies indicates that the Ca2+-betaine complex librates between two states of twisted deformation. In contrast with the Mn2+ spectra from BCCD, the staircase transitions were not faithfully indicated by the VO2+ spectra, being consistent with the absence of dielectric dispersions in VO2+-doped BCCD. The asymmetry in the VO2+ complex is considered to be responsible for strongly pinned condensates, resulting in no further staircase to be evolved otherwise. Although the whole transition scheme in BCCD except at 164 K suffers a modification, the VO2+ spectrum has provided clear evidence for a collective libration of Ca complexes below 164 K.
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