Order-disorder transitions in semiconductors represent a critical class of phase transformations governed by configurational entropy rather than displacive mechanisms. These transitions involve the rearrangement of atomic species on crystallographic sites, leading to changes in long-range or short-range order without significant distortion of the underlying lattice. A prototypical example occurs in the chalcopyrite-structured compound ZnSnP2, where cation ordering between Zn and Sn sites drives thermodynamic stability and modifies electronic properties.

The fundamental thermodynamics of order-disorder transitions derive from the balance between enthalpy and entropy contributions to the free energy. In ZnSnP2, the low-temperature phase exhibits perfect cation ordering with Zn and Sn occupying distinct 4a and 4b Wyckoff positions in the I-42d space group. As temperature increases, configurational entropy becomes increasingly dominant, promoting partial or complete randomization of cations across available sites. The transition temperature (T_c) marks the point where the entropic contribution (-TΔS) outweighs the enthalpic stabilization (ΔH) of the ordered state. For ZnSnP2, experimental studies place T_c near 945 K, above which the system adopts a disordered sphalerite-like structure with statistical cation distribution.

Neutron scattering techniques provide unparalleled insight into these transitions due to the method's sensitivity to atomic ordering and isotopic contrast. Diffuse neutron scattering measurements reveal the evolution of short-range order parameters through the transition, characterized by the Warren-Cowley parameters α_ij. In partially ordered ZnSnP2, neutron data shows persistent local preference for Zn-P and Sn-P bonding configurations even above T_c, evidenced by asymmetric peak broadening in reciprocal space. The total scattering structure factor S(Q) exhibits characteristic changes in superlattice reflections such as the (101) and (103) peaks, whose intensities decay according to a power law (I ∝ (T_c-T)^2β) as temperature approaches T_c from below.

Quantitative analysis of neutron diffraction patterns enables determination of the long-range order parameter η, defined as η = (r_A - x_A)/(1 - x_A) where r_A is the fraction of A-sites occupied by the "correct" cation and x_A is the stoichiometric fraction. For ZnSnP2, η follows a Landau-type temperature dependence with critical exponent β ≈ 0.31, consistent with a second-order transition in the three-dimensional Ising universality class. Inelastic neutron scattering further probes the dynamics of ordering through measurements of phonon density of states, revealing soft mode behavior associated with the loss of translational symmetry during disordering.

Entropy-driven disordering manifests in several measurable properties beyond structural parameters. The heat capacity C_p exhibits a λ-type anomaly at T_c with an excess entropy ΔS_dis ≈ 5.2 J/mol·K for complete disordering in ZnSnP2. Electrical resistivity measurements show an anomalous increase near T_c due to enhanced electron scattering from disordered cations, while optical bandgap measurements demonstrate a red-shift of ~0.15 eV upon complete disordering. These changes correlate directly with modifications in the electronic density of states observed through angle-resolved photoemission spectroscopy.

The kinetics of ordering transitions follow Arrhenius behavior with activation energies typically between 1-2 eV for III-V and II-IV-V2 compounds. Isothermal neutron scattering studies of ZnSnP2 reveal two distinct timescales: rapid initial establishment of short-range order (τ_1 ~ 10^2 s at 900 K) followed by slower evolution of long-range order (τ_2 ~ 10^4 s). The kinetic pathway often proceeds through nucleation of ordered domains with characteristic sizes of 5-20 nm, as evidenced by analysis of neutron powder diffraction peak widths.

Comparative studies across related compounds reveal systematic trends in ordering thermodynamics. The table below summarizes key parameters for selected materials:

Material T_c (K) ΔH (kJ/mol) ΔS (J/mol·K) β exponent
ZnSnP2 945 38 40 0.31
CuInSe2 980 42 43 0.33
CdGeAs2 850 35 41 0.30

These data demonstrate that the chalcopyrite-disorder transition enthalpy scales approximately with the squared difference in cation radii (ΔH ∝ (r_A - r_B)^2), while entropy changes remain relatively constant near the theoretical value for random mixing (ΔS ≈ Rln2 ≈ 5.76 J/mol·K per exchangeable site).

Practical implications of order-disorder phenomena extend to device performance and stability. In photovoltaic applications, disordered ZnSnP2 exhibits reduced minority carrier lifetimes due to increased trap state density, while ordered material shows superior conversion efficiency. Neutron scattering studies have guided optimization of annealing protocols to preserve desired order parameters during device fabrication, typically involving rapid quenching from temperatures just below T_c to freeze in the ordered state.

Recent advances in time-resolved neutron scattering enable investigation of non-equilibrium ordering dynamics under external stimuli. Pump-probe experiments using pulsed neutron sources have captured photoinduced disordering in ZnSnP2 on nanosecond timescales, revealing transient metastable states with partial order parameters. Such studies provide fundamental insight into the limits of thermodynamic control in semiconductor processing.

The universality of order-disorder phenomena across semiconductor classes - from chalcopyrites to perovskites to skutterudites - underscores the importance of entropy-driven transitions in materials design. Neutron scattering remains indispensable for characterizing these complex transformations, particularly through its ability to distinguish isoelectronic species and quantify short-range correlations. Future developments in high-flux neutron sources and advanced detectors promise further elucidation of the subtle interplay between atomic-scale disorder and macroscopic properties in functional semiconductors.