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  Berret, J.-F.; Meissner, M.; Watson, S. K.; Pohl, R. O.; Courtens, E.: Glasslike thermal properties and isotope effect in Rb1-x(NH4)xH2PO4 mixed crystals. Physical Review Letters 67 (1991), p. 93-96

10.1103/PhysRevLett.67.93