Modular Metal Chalcogenide Chemistry: Secondary Building Blocks as a Basis of the Silicate‐Type Framework Structure of CsLiU(PS4)2

The novel uranium thiophosphate CsLiU(PS₄)₂ has been synthesized by reacting uranium metal, Cs₂S, Li₂S, S, and P₂S₅ at 700 °C in an evacuated silica tube. The crystal structure was determined by single‐crystal X‐ray diffraction techniques. CsLiU(PS₄)₂ crystallizes in the rhombohedral space group R$\bar{3}$ c (a = 15.2797(7) Å; c = 28.778(2) Å, V = 5818.7(5) ų, Z = 18). The structure ofCsLiU(PS₄)₂ is a unique three‐dimensional U(PS₄)₂^2– framework with large tunnels with an approximate diameter of 6.6 Å running parallel to the crystallographic c axis. The tunnels are filled with Cs⁺ cations. The smaller Li⁺ cations are located at tetrahedral sites at the periphery of the channels. In the structure of CsLiU(PS₄)₂ the uranium atoms are coordinated by thiophosphate groups in a pseudotetrahedral fashion, and the PS₄ groups act as linear connectors. Topologically, CsLiU(PS₄)₂ may be regarded a chalcogenide analogue of silicate frameworks, with the uranium atoms and PS₄ groups replacing silicon and oxygen atoms. Alternatively, CsLiU(PS₄)₂ may be viewed as a coordination polymer, which is formed in salt melts by the solid state equivalent of the “self‐assembly” reactions in solution. Magnetic susceptibility measurements indicated Curie–Weiss‐type behavior between 4 K and 300 K. The μ_eff of 2.83 μ_B at 300 K is in agreement with an f² configuration of U⁴⁺.     

Synthese und Kristallstrukturen von α‐ und β‐Ba3(PS4)2 sowie von Ba3(PSe4)2

Synthesis and Crystal Structures of α‐, β‐Ba₃(PS₄)₂ and Ba₃(PSe₄)₂ Ba₃(PS₄)₂ and Ba₃(PSe₄)₂ were prepared by heating mixtures of the elements at 800 °C for 25 h. Both compounds were investigated by single crystal X‐ray methods. The thiophosphate is dimorphic and undergoes a displacive phase transition at about 75 °C. Both modifications crystallize in new structure types. In the room temperature phase (α‐Ba₃(PS₄)₂: P2₁/a; a = 11.649(3), b = 6.610(1), c = 17.299(2) Å, β = 90.26(3)°; Z = 4) three crystallographically independent Ba atoms are surrounded by ten sulfur atoms forming distorted polyhedra. The arrangement of the PS₄ tetrahedra, isolated from each other, is comparable with the formation of the SO₄^2? ions of β‐K₂SO₄. In β‐Ba₃(PS₄)₂ (C2/m; a = 11.597(2), b = 6.727(1), c = 8.704(2) Å; β = 90.00(3)°; Z = 2) the PS₄ tetrahedra are no more tilted along [001], but oriented parallel to each other inducing less distorted tetrahedra and polyhedra around the Ba atoms, respectively. Ba₃(PSe₄)₂ (P2₁/a; a = 12.282(2), b = 6.906(1), c = 18.061(4) Å; β = 90.23(3)°; Z = 4) is isotypic to α‐Ba₃(PS₄)₂ and no phase transition could be detected up to about 550 °C.     

Phenakite‐Type BeP2N4—A Possible Precursor for a New Hard Spinel‐Type Material

BeP₂N₄ was synthesized in a multi‐anvil apparatus starting from Be₃N₂ and P₃N₅ at 5 GPa and 1500 °C. The compound crystallizes in the phenakite structure type (space group R$\bar 3$ , no. 148) with a=1269.45(2) pm, c=834.86(2) pm, V=1165.13(4)×10⁶ pm³ and Z=18. As isostructural and isovalence‐electronic α‐Si₃N₄ transforms into β‐Si₃N₄ at high pressure and temperature, we studied the phase transition of BeP₂N₄ into the spinel structure type by using density functional theory calculations. The predicted transition pressure of 24 GPa is within the reach of today’s state of the art high‐pressure experimental setups. Calculations of inverse spinel‐type BeP₂N₄ revealed this polymorph to be always higher in enthalpy than either phenakite‐type or spinel‐type BeP₂N₄. The predicted bulk modulus of spinel‐type BeP₂N₄ is in the range of corundum and γ‐Si₃N₄ and about 40 GPa higher than that of phenakite‐type BeP₂N₄. This finding implies an increase in hardness in analogy to that occurring for the β‐ to γ‐Si₃N₄ transition. In hypothetical spinel‐type BeP₂N₄ the coordination number of phosphorus is increased from 4 to 6. So far only coordination numbers up to 5 have been experimentally realized (γ‐P₃N₅), though a sixfold coordination for P has been predicted for hypothetic δ‐P₃N₅. We believe, our findings provide a strong incentive for further high‐pressure experiments in the quest for novel hard materials with yet unprecedented structural motives.     

Motive dichtester Kugelpackungen: Die Verbindungen Zn3(PS4)2 und LiZnPS4

Motifs of Closest Packings: The Compounds Zn₃(PS₄)₂ and LiZnPS₄ The crystal structure of Zn₃(PS₄)₂ was determined by single crystal X‐ray methods. The compound crystallizes tetragonally (P4¯n2; a = 7.823(1), c = 9.053(1)Å; Z = 2) with a new structure type built up by corner‐sharing ZnS₄ tetrahedra, which form two‐dimensional layers. Between them the P atoms are coordinated likewise tetrahedrally by sulfur. The PS₄ tetrahedra are arranged according to the motif of the cubic closest packing with zinc in three quarters of the tetrahedral voids. LiZnPS₄ (I4¯; a = 5.738(1), c = 8.914(1)Å; Z = 2) was synthesized by heating the elements at 400 °C. In comparison with Zn₃(PS₄)₂ one Zn atom is replaced by two Li atoms. The metal atoms are located in the centres of the sulfur tetrahedra in such a way that the unit cell volume is only about half that of the zinc compound. In this packing of the PS₄ units all the tetrahedral voids are occupied by lithium and zinc atoms. Chemical bonding in LiZnPS₄ is discussed by means of the electron localization function ELF.     

Ab initio study of the high‐temperature phase transition in crystalline GeO2

Germanium dioxide (GeO₂) takes two forms at ambient pressure: a thermodynamically stable rutile‐type structure and a high‐temperature quartz‐type polymorph. Here, we investigate the phase stability at finite temperatures by ab initio phonon and thermochemical computations. We use gradient‐corrected density‐functional theory (PBE‐GGA) and pay particular attention to the modeling of the “semicore” germanium 3d orbitals (ascribing them either to the core or to the valence region). The phase transition is predicted correctly in both cases, and computed heat capacities and entropies are in excellent agreement with thermochemical database values. Nonetheless, the computed formation energies of α‐quartz‐type GeO₂ (and, consequently, the predicted transition temperatures) differ significantly depending on theoretical method. Remarkably, the simpler and cheaper computational approach produces seemingly better results, not worse. In our opinion, GeO₂ is a nice test case that illustrates both possibilities and limitations of modern ab initio thermochemistry. © 2013 Wiley Periodicals, Inc.     

Theoretical Investigation of the Solid State Reaction of Silicon Nitride and Silicon Dioxide forming Silicon Oxynitride (Si2N2O) under Pressure

The high‐pressure behavior of Si₂N₂O is studied for pressures up to 100 GPa using density functional theory calculations. The investigation of a manifold of hypothetical polymorphs leads us to propose two dense phases of Si₂N₂O, succeeding the orthorhombic ambient‐pressure polymorph at higher pressures:a defect spinel structure at moderate pressures and a corundum‐type structure at very high pressures. Taking into account the formation of silicon oxynitride from silicon dioxide and silicon nitride and its pressure dependence, we propose five pressure regions of interest for Si₂N₂O within the pseudo‐binary phase diagram SiO₂‐Si₃N₄: (i) stability of the orthorhombic ternary phase of Si₂N₂O up to 6 GPa, (ii) a phase assemblage of coesite, stishovite, and β‐Si₃N₄ between 6 and 11 GPa, (iii) a possible defect spinel modification of Si₂N₂O between 11 and 16 GPa, (iv) a phase assemblage of stishovite and γ‐Si₃N₄ above 40 GPa, and (v) a possible ternary Si₂N₂O phase with corundum‐type structure beyond 80 GPa. The existence of both ternary high‐pressure phases of Si₂N₂O, however, depends on the delicate influence of configurational entropy to the free energy of the solid state reaction.     

K3Cr2(PS4)3: A New Chromium Thiophosphate with a One‐Dimensional [Cr2(PS4)]3— Anion Chain

A new chromium thiophosphate, K₃Cr₂(PS₄)₃ has been prepared and characterized by single‐crystal diffraction, temperature dependent magnetic susceptibility measurements and optical spectroscopy. K₃Cr₂(PS₄)₃ crystallizes in the monoclinic space group P2₁/n (No. 14) with a = 9.731(2) Å, b = 11.986(2) Å, c = 17.727(4) Å, β = 96.52(2)°, V = 2054.2(2) ų, Z = 4, and R = 0.044. The anionic part of the structure consists of dimeric Cr₂(μ₃‐S₃PS)₂ units which are linked by bidentate PS₄ groups to form infinite one‐dimensional [S₂P_S2Cr₂(μ₃S₃P_S)₂]^3— chains separated by K⁺ cations. The Cr^III centers of the Cr₂(μ₃‐S₃PS)₂ units are antiferromagnetically coupled. The magnetic susceptibility data may be fitted using a D‐Heisenberg model for S = 3/2 with g = 2.02 and J/k = 10K. K₃Cr₂(PS₄)₃ is semiconducting with an optical band gap of 1.35 eV.     

Drei Alkalimetall‐Erbium‐Thiophosphate: Von der Schichtstruktur bei KEr[P2S7] zur dreidimensionalen Vernetzung in NaEr[P2S6] und Cs3Er5[PS4]6

Three Alkali‐Metal Erbium Thiophosphates: From the Layered Structure of KEr[P₂S₇] to the Three‐Dimensional Cross‐Linkage in NaEr[P₂S₆] and Cs₃Er₅[PS₄]₆ The three alkali‐metal erbium thiophosphates NaEr[P₂S₆], KEr[P₂S₇], and Cs₃Er₅[PS₄] show a small selection of the broad variety of thiophosphate units: from ortho‐thiophosphate [PS₄]^3? and pyro‐thiophosphate [S₃P–S–PS₃]^4? with phosphorus in the oxidation state +V to the [S₃P–PS₃]^3? anion with a phosphorus‐phosphorus bond (d(P–P) = 221 pm) and tetravalent phosphorus. In spite of all differences, a whole string of structural communities can be shown, in particular for coordination and three‐dimensional linkage as well as for the phosphorus‐sulfur distances (d(P–S) = 200 – 213 pm). So all three compounds exhibit eightfold coordinated Er³⁺ cations and comparably high‐coordinated alkali‐metal cations (CN(Na⁺) = 8, CN(K⁺) = 9+1, and CN(Cs⁺) ≈ 10). NaEr[P₂S₆] crystallizes triclinically ( ; a = 685.72(5), b = 707.86(5), c = 910.98(7) pm, α = 87.423(4), β = 87.635(4), γ = 88.157(4)°; Z = 2) in the shape of rods, as well as monoclinic KEr[P₂S₇] (P2₁/c; a = 950.48(7), b = 1223.06(9), c = 894.21(6) pm, β = 90.132(4)°; Z = 4). The crystal structure of Cs₃Er₅[PS₄] can also be described monoclinically (C2/c; a = 1597.74(11), b = 1295.03(9), c = 2065.26(15) pm, β = 103.278(4)°; Z = 4), but it emerges as irregular bricks. All crystals show the common pale pink colour typical for transparent erbium(III) compounds.     

Preparation of miktoarm star‐block copolymers PSn‐b‐PVAc4‐n via combination of ATRP and RAFT polymerization

Three tetrafunctional bromoxanthate agents (Xanthate₃‐Br, Xanthate₂‐Br₂, and Xanthate‐Br₃) were synthesized. Initiative atom transfer radical polymerizations (ATRP) of styrene (St) or reversible addition fragmentation chain transfer (RAFT) polymerizations of vinyl acetate (VAc) proceeded in a controlled manner in the presence of Xanthate₃‐Br, Xanthate₂‐Br₂, or Xanthate‐Br₃, respectively. The miktoarm star‐block copolymers containing polystyrene (PS) and poly(vinyl acetate) (PVAc) chains, PS_n‐b‐PVAc_4‐n (n = 1, 2, and 3), with controlled structures were successfully prepared by successive RAFT and ATRP chain‐extension experiments using VAc and St as the second monomers, respectively. The architecture of the miktoarm star‐block copolymers PS_n‐b‐PVAc_4‐n (n = 1, 2, and 3) were characterized by gel permeation chromatography and ¹H NMR spectra. Furthermore, the results of the cleavage of PS₃‐b‐PVAc and PVAc₂‐b‐PS₂ confirmed the structures of the obtained miktoarm star‐block copolymers. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

The effects of atom pyramidalization and square distribution on the stability of F4F6‐(BN)n polyhedrons

The structures and stability of F₄F₆‐(BN)_n polyhedrons (n = 20–30) with the alternation of B and N atoms were studied with DFT method. The calculation results reveal that the atoms at square–square fusions with large pyramidalization angles are remarkably extruded out of the surfaces of (BN)_n polyhedrons. The energetically favorable isomers do not contain square–square bonds and the energies of those isomers containing square–square bonds increase with the number of square–square bonds linearly, demonstrating that the energetically favorable structures of F₄F₆‐(BN)_n polyhedrons satisfy the isolated square rule and square adjacency penalty rule. The atom pyramidalization determines the stability of the isomers. The binding energy is fitted to the numbers of vertices formed from different faces and a model is proposed to predict the relative stability of these polyhedral molecules. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Strain state of poly(N‐isopropylacrylamide) in polystyrene‐b‐poly(N‐isopropylacrylamide) block copolymers and binary blends with polystyrene

Three diblock copolymers of polystyrene‐b‐poly(N‐isopropylacrylamide) (PS‐b‐PNIPAM) were prepared by reversible addition‐fragmentation chain transfer technique (RAFT) with compositions f_PS = 0.84, f_PS = 0.29, and f_PS = 0.33. Block copolymers rich in PNIPAM were blended with polystyrene and its morphological effects were studied. The morphology of thin films was induced by acetone vapor and determined in the dried state by means of TEM. Copolymers with f_PS = 0.84 and f_PS = 0.29 form hexagonally packed cylinder (HPC) morphologies while that with f_PS = 0.33 corresponds to a lamellar structure. In almost all cases where PNIPAM constitutes the continuous phase, a contraction of the PNIPAM blocks with respect to their average unperturbed dimension was observed, contrary to what one expects from the physics of self‐assembly of block copolymers. In contrast, for HPC morphology where PNIPAM is confined in a PS matrix, both blocks are highly extended. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1368–1376     

K2NdP2S7: A Mixed‐Valent Neodymium(III) Thiophosphate According to K4Nd2[PS4]2[P2S6] with Discrete [PS4]3– and [S3P–PS3]4– Anions

Pale blue, lath‐shaped single crystals of K₂NdP₂S₇ (≡ K₄Nd₂[PS₄]₂[P₂S₆]; monoclinic, P2₁/n, a = 904.76(8), b = 677.38(6), c = 1988.7(2) pm, β = 97.295(5)°, Z = 2) are obtained by the reaction of Nd, S and P₂S₅ with an excess of KCl as a flux in evacuated silica tubes at 750 °C (7 d) which should produce Nd[PS₄] instead. Beside isolated [PS₄]^3– tetrahedra, the crystal structure contains discrete ethane‐analogous [P₂S₆]^4– (≡ [S₃P–PS₃]^4–) units in staggered conformation with tetravalent phosphorus cations and a P–P distance of 219 pm. The two crystallographically different potassium cations show coordination numbers of nine and ten in the shape of distorted mono‐ and bicapped square antiprisms. Finally, the Nd³⁺ cations are surrounded by eight sulfur atoms arranged as (uncapped) square antiprisms. The entire structure is dominated by (K1)⁺ containing {(Nd₂[PS₄]₂[P₂S₆])^4–} layers parallel (101) which are three‐dimensionally interconnected by (K2)⁺ cations.     

Simulation study on the coil‐globule transition of adsorbed polymers

Coil‐globule transition of adsorbed polymers on attractive surface is simulated by using dynamic Monte Carlo simulation. The effect of surface attraction strength E_PS and intrachain attraction strength E_PP on polymer phases is investigated. The coil‐globule transition point is dependent on E_PS, while the globule conformation is dependent on both E_PS and E_PP. At small E_PS, the conformation of adsorbed polymer is three‐dimensional layer structure. While at large E_PS, the conformation of adsorbed polymer is roughly two‐dimensional (2D) at E_PP = 0, and we observe a 2D coil‐globule transition at E*_PP and a layer‐forming transition from 2D conformation to three‐dimensional layer structure at E*_PP,L > E*_PP. The layer‐forming transition point E*_PP,L increases with E_PS as E*_PP,L = E_PS ? 1.4. In addition, we find that the adsorption suppresses the coil‐globule transition, i.e., the coil‐globule transition point E*_PP increases with the increase in E_PS. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2359–2367     

The Crystal Structure of ζ2‐Al3Cu4‐δ

The crystal structure of the ζ₂‐phase Al₃Cu_4‐δ was determined by means of X‐ray powder diffraction: a = 409.72(1) pm, b = 703.13(2) pm, c = 997.93(3) pm, space group Imm2, Pearson symbol oI24‐3.5, R_I = 0.0696. ζ₂‐Al₃Cu_4‐δ forms a distinctive a × √3a × 2c superstructure of a metal deficient Ni₂In‐type‐related structure. The phase is meta‐stable at ambient temperature. Between 400 °C and 450 °C it decomposes into ζ₁‐Al₃Cu₄ and η₂‐AlCu. Entropic contributions to the stability of ζ₂‐Al₃Cu_4‐δ are reflected in three statistically or partially occupied sites.     

New compound in the system Sc-Cr-B

The isothermal section of the phase diagram of Sc-Cr-B system at 800°C is constructed using the data of X-ray analysis. CrB₂ dissolves up to 5 mol% of ScB₂, and ScB₂ dissolves up to about 20 mol% of CrB₂. A new boride Sc₂CrB₆ was found and its crystal structure was established by single-crystal X-ray diffraction: Y₂ReB₆ type, space group Pbam, Z=4, a=8.7909(4), b=11.0541(6), c=3.2996(1)Å, diffractometer Kappa CCD-Nonius, MoKα, R_F=0.047, R_w=0.064.     

Crystal Structure and Vibrational Spectrum of Thallium(I) Europium(II) Tetrathiophosphate(V), TIEu[PS4]

TlEu[PS₄] was synthesized from the elements in a sealed quartz ampoule at 1 150 K. The compound forms transparent orange needles, stable in air and moisture. It crystallizes in the orthorhombic system, space group Pnma (No. 62), with cell dimensions a = 12.157(2), b = 6.581(1), c = 8.802(2) Å, Z = 4. The crystal structure consists of discrete [PS₄]^3? anions interconnected by Tl⁺ and Eu²⁺. The tetrahedral [PS₄]^3? groups are slightly distorted with P? S bond lengths in the range 2.028 to 2.043 Å. These tetrahedral anions are arranged in such a way that the sulfur atoms form columns of distorted trigonal S₆ prisms along [0 0 1]. The columns are condensed via common edges to puckered layers parallel to (1 0 0). The interlayer region consists of empty distorted half-cubes and tetrahedral holes, half of them filled by P atoms. The trigonal prisms in the columns are centered alternately by Tl⁺ and Eu²⁺. In this way, the structure can be regarded as an ordered superstructure of the InNi₂ type, where half of the tetrahedral holes are filled by phosphorus atoms: InInNi₄□ ? TlEuS₄P□. TlEu[PS₄] is a centrosymmetric variant of the TlSn[PS₄] structure type (space group Pna2₁). The vibrational spectrum is in accordance with the X-ray crystal structure, the Raman and infrared vibrations are assigned on the basis of [PS₄]^3? units with C_2v symmetry.     

Sur le diborure et le tétraborure de magnésium. Considérations cristallochimiques sur les tétraborures

MgB₂ and MgB₄ have been prepared at high temperature in sealed molybdenum vessels from mixtures of the elements. The utilization of an excess of metal in the vessel generates a pressure of magnesium vapor which inhibits thermal decomposition of the compounds during the synthesis. The structure of MgB₄ has been established from single crystal data collected on an automatic diffractometer. MgB₄ is orthorhombic, space group Pnam with a = 5.464; b = 7.472; c = 4.428Å and Z = 4. The structure of MgB₄, is based on chains of boron pentagonal pyramids in which the averaged BB bond is 1.787 Å. Interchain BB bonds of 1.730 Å are responsible for the three-dimensional boron framework. The Mg atoms, located in tunnels, form zigzag chains. The structure of MgB₄ is compared to those of ThB₄ and CrB₄; it is concluded that the size of the metal atom plays an important role in the nature of the boron framework which exists. In MgB₄, the fundamental unit of the boron skeleton is a pentagonal pyramid; a new feature, in boron-rich borides where this type of coordination polyhedron was previously found only in B₁₂ icosahedra.     

Nitride Spinel: An Ultraincompressible High‐Pressure Form of BeP2N4

Owing to its outstanding elastic properties, the nitride spinel γ‐Si₃N₄ is of considered interest for materials scientists and chemists. DFT calculations suggest that Si₃N₄‐analog beryllium phosphorus nitride BeP₂N₄ adopts the spinel structure at elevated pressures as well and shows outstanding elastic properties. Herein, we investigate phenakite‐type BeP₂N₄ by single‐crystal synchrotron X‐ray diffraction and report the phase transition into the spinel‐type phase at 47 GPa and 1800 K in a laser‐heated diamond anvil cell. The structure of spinel‐type BeP₂N₄ was refined from pressure‐dependent in situ synchrotron powder X‐ray diffraction measurements down to ambient pressure, which proves spinel‐type BeP₂N₄ a quenchable and metastable phase at ambient conditions. Its isothermal bulk modulus was determined to 325(8) GPa from equation of state, which indicates that spinel‐type BeP₂N₄ is an ultraincompressible material.     

Molecular Propellers that Consist of Dehydrobenzo[14]annulene Blades

A new class of propel‐ ler‐shaped compound ( 4 ), which consisted of dehydrobenzo[14]annulene ([14]DBA) blades, as well as its naphtho homologues ( 5 and 6 ), have been prepared. Although NMR studies of compound 4 did not provide useful information regarding its conformation in solution, DFT calculations with different functionals and the 6‐31G* basis set all indicated that the D₃‐symmetric structure was energetically more favorable than the C₂ conformer. From X‐ray crystallographic analysis, it appeared that compound 4 adopted a propeller‐shaped‐, approximately D₃‐symmetric structure in the solid state, in which the [14]DBA blades were twisted substantially owing to steric repulsion between the neighboring benzene rings. On the contrary, in the case of compound 6 , although the DFT calculations with the B3LYP functional predicted that the D₃‐symmetric conformation was more stable, calculations with the M05 and M05‐2X functionals indicated that the C₂ conformer was more favorable because of π–π interactions between the naphthalene units of a pair of neighboring blades. Indeed, X‐ray analysis of compound 6 showed that it adopted an approximately C₂‐symmetric conformation. Moreover, on the basis of variable‐temperature ¹H NMR measurements, we found that compound 6 adopted a C₂ conformation and the barrier for interconversion between the C₂–C₂ conformers was estimated to be 16.2 kcal mol^?1; however, no indication of the presence of the D₃ isomer was obtained. The relatively small energy barriers to interconversion, despite the large overlapping of neighboring blades, was ascribed to the flexibility of the acetylene linkages, which could be deformed substantially in the transition state of the ring‐flip.     

Chemical Modification and Energetically Favorable Atomic Disorder of a Layered Thermoelectric Material TmCuTe2 Leading to High Performance

Thermoelectric (TE) materials have continuously attracted interest worldwide owing to their capability of converting heat into electricity. However, discovery and design of new TE material system remains one of the greatest difficulties. A TE material, TmCuTe₂, has been designed by a substructure approach and successfully synthesized. The structure mainly features CuTe₄‐based layers stacking along the c axis that are separated by Tm³⁺ cations. Such an intrinsic Cu site vacancy structure undergoes a first‐order phase transition at around 606 K driven by the energetically favorable uniform Cu atom re‐distribution on the covalent CuTe₄‐based layer substructure, as shown by crystal structure simulations and variable‐temperature XRD data. Featured with very low thermal conductivity (ca. 0.6 W m^?1 K^?1), large Seebeck coefficient (+185 μV K^?1), and moderate electrical conductivity (220 S cm^?1), TmCuTe₂ has a maximum ZT of 0.81 at 745 K, which is nine times higher than the value of 0.09 for binary Cu₂Te, thus making it a promising candidate for mid‐temperature TE applications. Theoretical studies uncover the electronic structure modifications from the metallic Cu₂Te to the narrow gap semiconductor TmCuTe₂ that lead to such a remarkable performance enhancement.