VOLUME 19, NUMBER 3 PHYSICAL REVIEW LETTERS 17 JULY 1967 NEAR INSTABILITY OF LATTICE RESONANT MODES* B. P. Clayman, L G. Nolt, and A. J. Sievers Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, New York (Received 29 May 1967) Upon alloying KBr with other alkali halides, we have observed a 100% shift in the frequen- cy of the lithium-activated KBr resonant mode. This anomalous behavior indicates that the res- onant mode frequency will approach 0 for a nominal increase in KBr lattice constant and is cogent experimental evidence of the incip- ient instability associated with such low-lying modes. Infrared-active lattice resonant modes have been observed as absorption lines in the far- infrared spectral region for a large number of lattice-defect systems. 1 Many of these ab- sorption lines have been identified with the qua- silocalized lattice modes which occur when the lattice impurity has a stable equilibrium position at the normal lattice site. 2 In partic- ular, uniaxial stress experiments on the sharp absorption line centered at 16.2 cm" 1 in KBr:Li + indicate that the lithium impurity in this sys- tem occupies the normal lattice site. 3 More recent studies of the stress-induced frequen- cy shifts for the lattice resonant modes in KI;Ag + (17.3 cm- 1 ) and NaCl:Cu + (23.4 cm" 1 ) show similar characteristics. 4 There are other systems, however, in which the impurity is thought to occupy a position displaced from the normal lattice site. KCl'.Li"" is undoubtedly the most thoroughly explored system of this type. 5 More recently, the off- center configuration has been proposed for a number of other lattice-defect systems. 6 Thus, there emerges the present picture of at least two distinct classes of lattice-impurity modes, those associated with the impurity at the nor- mal lattice site (class I) and those with an off- center configuration (class II). If this picture is correct, then a continuous change in some as-yet-undefined parameter should give rise to a transition between the different classes. The large hydrostatic coupling coefficients obtained from our stress measurements 3 ' 4 im- ply that large frequency shifts occur for small changes in the lattice constant, and that the lattice constant might be an appropriate param- eter with which to induce a transition between two classes. The average lattice constant of an alloy of two alkali halides has a value inter- mediate to the lattice constants of the two con- .+ stituents. This method of varying the lattice constant has been exploited to study F- center 7 and U-center 8 systems. Thus by alloying KI or KC1 with KBr:Li + , it is possible to increase or decrease the average lattice constant over an appreciable range in a predictable fashion. 9 We have measured the frequency shift of the lithium-induced resonant mode as a function of this average lattice constant. Typical far-infrared absorption spectra for two of the lithium-doped KBr alloys are shown in Fig. 1. In general, the center frequency of the absorption shifts to higher frequencies and the line broadens with the addition of KC1 or NaBr; it shifts to lower frequencies and broad- ens upon the addition of KI. For KC1 concen- trations from 0 to 8 mole%, the integrated ab- Photon Energy (mev) 2 3 4 E ' o o> 0| o 8 is o I I0| in o S 5 < 0 0.2 0.1 0, ~i— ! r KBr+10.2% KCI + .0I6% LiBr KBr+.017% Li Br ]ki = 16.2 cm-l KBr + 4.5 %K I +.01% Li Br = 11.2 cm" 10 20 30 Frequency (crrH) FIG. 1„ Lithium-induced absorption in two KBr al- loys compared with that of KBr: LiBr at 4.2°K. Concen- trations are expressed in mole% as determined by chemical analysis. The frequency of the centroid of the absorption line is denoted by oo c . Note the differ - ent absorption-coefficient scales for the three cases. These data were obtained using a lamellar-type inter- ferometer and a recently developed 0.4°K Ge bolome- ter. [H. D. Drew and A. J. Sievers, Bull. Am. Phys. Soc. JJ2, 77 (1967).] The instrumental resolution was 0.5 cm"" 1 for the two alloy samples and 0.2 cm"" 1 for the KBr: LiBr sample. Ill