P31Chemical-Shift Study of the Ferroelectric Transition in KD

NMR In Disordered ferroelectrics and deuteron glasses

“…T > T, : (RO.H)symm = 1.255 A (13) and would contradict the experimental data (i.e. expression (12)).…”

Magic angle spinning NMR on single crystals as a new aid in characterizing phase transitions: application to squaric acid

“…It utilized measurement on single crystals, as a function of their orientation in three mutually perpendicular planes to obtain the principal components of the chemical shift tensor, δ, and then to deduce δ ISO as the average (i.e., tensor trace) of the principal components. This procedure, however, yields unacceptably large errors in the magnitude of δ ISO , essentially because δ is a highly anisotropic tensor quantity and its trace is a relatively small number [1][2][3][4][5]. This is evident, for example, from earlier 13 C NMR studies [3,4] of squaric acid, H 2 C 4 O 4 , whose structure is shown in Fig.…”

“…These authors recognized the fact that δ ISO , the isotropic part of the chemical shift tensor of any given nucleus, is a sensitive function of the electronic distribution over the whole molecular unit, hence the detection of any variation in this parameter would be a direct evidence of a change in the electronic structure of the fragment being investigated [2][3][4][5][6]. Thus the temperature dependence of δ ISO could serve as a sensitive microscopic probe of the minute atomic displacements related to a phase transition, and also of such characteristics as the 'displacive' versus 'order-disorder' behavior of the investigated lattice [2][3][4]. For solids, however, accurate experimental measurement of δ ISO is rather difficult.…”

“…Pioneering work in this area has been reported by Blinc [2] and later by Mehring [3,4]. These authors recognized the fact that δ ISO , the isotropic part of the chemical shift tensor of any given nucleus, is a sensitive function of the electronic distribution over the whole molecular unit, hence the detection of any variation in this parameter would be a direct evidence of a change in the electronic structure of the fragment being investigated [2][3][4][5][6].…”

High-precision31P chemical shift measurements on KH2PO4-type crystals: role of electronic instability in the ferroelectric transition mechanism

“…. O bond, or whether electronic instabilities are simultaneously involved in the transition mechanism [1,2]. It appears that the theoretical process has been hampered by the lack of experimental techniques [1] that can clearly distinguish between the order-disorder versus displacive mechanisms [1].…”