Orientational Freezing in KCN-KBr Studied by Magnetic Resonance

Doverspike, Montee A.; Wu, Meng-Chou; Conradi, Mark S.

doi:10.1103/physrevlett.56.2284

Cited by 24 publications

(5 citation statements)

References 31 publications

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“…9,11,12 This anomaly has been interpreted as evidence for the formation of a quadrupolar glass state.11, 13 There is now evidence that, in these systems, dipolar (head-to-tail) and quadrupolar (orientational) freezing temperatures differ. 14,15 In order to rationalize the effect of dilution on the formation of an orientational glass state, theoretical work has invoked the coupling of the rotational degrees of freedom of the CN" anions to the random strain fields generated by the chemical disorder of the anion sublattice. 16 We have reached similar conclusions on the basis of computer simulation studies which indicated that the ground-state structure of (KBr),_x(KCN)x and related systems was determined principally by a competition between translation-rotation and strain-rotation couplings.17…”

Section: Introductionmentioning

confidence: 99%

Quadrupolar freezing in potassium bromide cyanide ((KBr)1-x(KCN)x) mixed crystals

Lewis¹,

Klein²

1987

J. Phys. Chem.

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We have used constant-pressure molecular dynamics to investigate the process of quadrupolar freezing in (KBr),_x(KCN)x mixed crystals. In agreement with experiment, a system with x = 0.25 was seen to remain cubic at all temperatures and form an orientational glass. The ground state is analyzed in terms of absolute and relative ordering of the CN" molecular anions. We find that neighboring quadrupoles exhibit a definite preference for ferroelastic ordering, though a broad distribution of relative orientations is also present. These results are in sharp contrast to those in a system with x = 0.73, which is unequivocally found to freeze into an orientationally ordered (crystal) state at low temperature.

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Section: Introductionmentioning

confidence: 99%

Quadrupolar freezing in potassium bromide cyanide ((KBr)1-x(KCN)x) mixed crystals

Lewis¹,

Klein²

1987

J. Phys. Chem.

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“…At all temperatures below 32 K, stimulated echoes in 5/55/40 have a decay constant longer than 2 s, a factor of lo5 longer than TZRL Thus, if the homogeneous distribution of 7, model is correct, the distribution must be exceedingly broad. At 16 K, the decay time is longest (12 s); this is the same temperature as the maximum in T2 and minimum in TI (see below): Stimulated echoes at long times also can be formed in Hz (33,38) and in KCN/KBr (21).…”

Section: Resultsmentioning

confidence: 99%

“…1 are presented I3C nrnr spectra from CO/N2/Ar (20140140) along with 15N spectra from KCN/KBr (50150) for comparison (21). In both cases the linewith is dominated by the chemical shift anisotropy of the molecule, a uniaxially symmetric tensor.…”

Section: Resultsmentioning

confidence: 99%

CO/N₂/Ar orientational glass: magnetic resonance

Walton¹,

Wu²,

Conradi³

1988

Can. J. Chem.

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. J. Chem. 66, 680 (1988).The nrnr lineshapes and linewidths of KCN/KBr, N2/Ar, and CO/N2/Ar are compared at corresponding states and are found to agree closely. This indicates a similar kind of orientational freezing, despite major differences in the anisotropic molecular interactions. The line narrowing and T2 data are explicable by two different theories. The one involves an orientation probability function, while the other invokes a broad homogeneous distribution of correlation times. Spin-lattice relaxation measurements indicate that T I and dielectric loss are controlled at low temperatures by nearly n flips of CO molecules. On a compare les formes et les largeurs des raies de rmn du KCN/KBr, du N2/Ar et du CO/N2/Ar dans des Ctats correspondants et on a trouvt qu'ils sont trks semblables. Ce resultat indique que, malgre des differences majeures dans les interactions molCculaires anisotropes, leurs orientations lors de leurs congelations sont semblables. Les donnies relatives au retricissement des bandes et aux valeurs de T2 peuvent Ctre expliquees par deux thCories differentes. La premiere implique une fonction de probabilite dand l'orientation alors que l'autre fait appel B une distribution homogene large des temps de correlation. Des mesures de relaxation spin-rkseau indiquent que la valeur de T I ainsi que la perte dielectrique sont contr6lCes, B basse temperature, par pratiquement n inversions des molecules de CO.[Traduit par la revue]

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“…The latter has been interpreted as evidence for the formation of a quadruplar glass state.46,110 It is generally accepted that, in these systems, dipolar (head-to-tail) and quadrupolar (orientational) freezing temperatures differ. 111,112 In one of the earliest applications of the constantpressure MD method, Impey, Klein, and Sprik investigated the cubic -*• orthorhombic transition in pure KCN. 113 In their study, the CN" charge distribution was represented by a three-site model, with charges of -0.8 e at the N site, +0.8 e in the C-N bond a distance 0.204 A from the C atom, and -e outside the C-N bond a distance 0.222 A from the C atom.…”

Section: Structural Transitionsmentioning

confidence: 99%

“…The observation of a low-temperature monoclinic phase accords well with experiment and is by itself a gratifying result, especially when we recall that the ground-state of pure KCN is orthorhombic. However, the monoclinic phase is experimentally preceded by a rhombohedral phase which forms at 112 K and whose range of existence is only about 4 K. Accordingly, a more careful study of the same site-disordered sample, this time using much smaller temperature intervals on cooling, was carried out. Results are shown in Figure 17b.…”

Section: Structural Transitionsmentioning

confidence: 99%