We investigate an influence of the crystal structure imperfections on the electronic properties and dielectric functions of the In 0.5 Tl 0.5 I semiconductor in the frame of the density functional theory calculations. The tensor of electron effective mass m à ij for the InI, In 0.5 Tl 0.5 I and TlI crystals has been calculated for the valence and conduction bands at different K-points of the Brillouin zone. The dielectric functions e(hm) of the imperfect crystals based on In 0.5 Tl 0.5 I solid state solution with iodine vacancy and a thallium interstitial atom were calculated taking into consideration the inter-band and intra-band electron transitions. The studies of the imperfect crystals reveal increased low-frequency and stationary electron conductivity with anisotropy resulted from the anisotropy of the electron effective mass tensor. Our findings explain the origin of crucial changes in the band structure by formation of the donor half-occupied levels close to the unoccupied conduction bands due to the crystal structure defects, i.e., iodine vacancy or a thallium interstitial atom. It has been shown that in the case of real crystals, in particular metal-halides, the proper consideration of defects in quantum-chemical calculations results in a better matching with experimental data and, opposite to the perfect structure calculations, gives opportunities to explain the observed phenomena.

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The continuous phase transitions are observed in the crystals with incommensurate phase. They are transitions of parent-incommensurate phases (Ti); transitions between metastable states; incommensurate-commensurate phases (T c ). It was set that phase transition between parent and incommensurate phase is a continuous second-order phase transition with a critical index β = 0.5. The transition between metastable states is a continuous phase transition through the intermediate temperature region -incommensurate phase. The wave vector changes with the temperature here and wave vector q * = q 1 − q 2 appears, where q 1 , q 2 denote commensurate values of incommensurability wave vector of neighboring metastable states. It was shown that the phase transition between incommensurate and commensurate phases is a continuous phase transition.

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У статтi подано результати ab initio розрахункiв зонно-енерґетичного спектра кристала (N(CH3)4)2ZnCl4. У межах методу псевдопотенцiалу теоретично вивчено динамiку змiни параметрiв електронної пiдсистеми у кристалi (N(CH3)4)2ZnCl4. Установлено, що заборонена щiлина кристала має прямозонний характер. На основi щiльностi станiв установлено генезис зон провiдностi та валентної. З розрахунку електронного енерґетичного спектра визначено ефективнi маси електрона та дiрки. Отриманi результати задовiльно корелюють з експериментальними даними.Ключовi слова: електронний енерґетичний спектр, щiльнiсть станiв, ефективна маса, повна енерґiя.