We present the first observations of near-surface critical phenomena of a phase transition in a molecular solid showing both an order-disorder and a strong displacive component. The surface x-ray scattering experiments were performed on an ${\mathrm{NH}}_4$Br single crystal (${\mathit{T}}_{\mathit{c}}$=235 K). For the order parameter they show a crossover from a power law with the bulk critical exponent β=0.3±0.05 to a surface behavior with exponent ${\mathrm{\beta }}_1$=0.8±0.1. From the diffuse intensity we have determined the critical exponents ${\mathrm{\eta }}_{\mathrm{\parallel }}$=1.3±0.15, ν=0.5±0.15, and the correlation length ${\xi }_0^{+}$=4±0.5 Å.

• Received 16 March 1993

DOI:https://doi.org/10.1103/PhysRevLett.71.1188