Distortions from the Tetragonal HfCuSi<sub>2</sub>-Type Structures to the Orthorhombic GdCuAs<sub>1.15</sub>P<sub>0.85</sub>-Type Structures in <i>REMX</i><sub>2</sub> (<i>RE</i> = Rare Earth; <i>M</i> = Cu, Ag; <i>X</i> = P, As, Sb): Mismatch between the Sizes of the <i>M</i> and <i>X</i> Atoms