TWO-PHONON BOUND STATES IN AMMONIUM CHLORIDE AT FINITE TEMPERATURES A.A. Anikiev, B.S. Umarov, V.S. Gorelik et J.F. Vetelino J. Phys. Colloques, 42 C6 (1981) C6-152-C6-154 DOI: 10.1051/jphyscol:1981646