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The novel quaternary thiogermanate Li4CdGe2S7 (tetralithium cadmium digermanium heptasulfide) was discovered from a solid-state reaction at 750 °C. Single-crystal X-ray diffraction data were collected and used to solve and refine the structure. Li4CdGe2S7 is a member of the small, but growing, class of I4–II–IV2–VI7 diamond-like materials. The compound adopts the Cu5Si2S7 structure type, which is a derivative of lonsdaleite. Crystallizing in the polar space group Cc, Li4CdGe2S7 contains 14 crystallographically unique ions, all residing on general positions. Like all diamond-like structures, the compound is built of corner-sharing tetrahedral units that create a relatively dense three-dimensional assembly. The title compound is the major phase of the reaction product, as evidenced by powder X-ray diffraction and optical diffuse reflectance spectroscopy. While the compound exhibits a second-harmonic generation (SHG) response comparable to that of the AgGaS2 (AGS) reference material in the IR region, its laser-induced damage threshold (LIDT) is over an order of magnitude greater than AGS for λ = 1.064 µm and τ = 30 ps. Bond valence sums, global instability index, minimum bounding ellipsoid (MBE) analysis, and electronic structure calculations using density functional theory (DFT) were used to further evaluate the crystal structure and electronic structure of the compound and provide a comparison with the analogous I2–II–IV–VI4 diamond-like compound Li2CdGeS4. Li4CdGe2S7 appears to be a better IR nonlinear optical (NLO) candidate than Li2CdGeS4 and one of the most promising contenders to date. The exceptional LIDT is likely due, at least in part, to the wider optical bandgap of ∼3.6 eV.

The new quaternary thiosilicate, Li2PbSiS4 (dilithium lead silicon tetrasulfide), was prepared in an evacuated fused-silica tube via high-temperature, solid-state synthesis at 800 °C, followed by slow cooling. The crystal structure was solved and refined using single-crystal X-ray diffraction data. By strict definition, the title compound crystallizes in the stannite structure type; however, this type of structure can also be described as a compressed chalcopyrite-like structure. The Li+ cation lies on a crystallographic fourfold rotoinversion axis, while the Pb2+ and Si4+ cations reside at the intersection of the fourfold rotoinversion axis with a twofold axis and a mirror plane. The Li+ and Si4+ cations in this structure are tetrahedrally coordinated, while the larger Pb2+ cation adopts a distorted eight-coordinate dodecahedral coordination. These units join together via corner- and edge-sharing to create a dense, three-dimensional structure. Powder X-ray diffraction indicates that the title compound is the major phase of the reaction product. Electronic structure calculations, performed using the full potential linearized augmented plane wave method within density functional theory (DFT), indicate that Li2PbSiS4 is a semiconductor with an indirect bandgap of 2.22 eV, which compares well with the measured optical bandgap of 2.51 eV. The noncentrosymmetric crystal structure and relatively wide bandgap designate this compound to be of interest for IR nonlinear optics.

The isostructural crystal structures of Cu4FeGe2S7 and Cu4CoGe2S7 were solved and refined. All the metal cations form MS4 tetra­hedra and share corners to create a three-dimensional, non-centrosymmetric structure., The quaternary thio­germanates Cu4FeGe2S7 (tetra­copper iron digermanium hepta­sulfide) and Cu4CoGe2S7 (tetra­copper cobalt digermanium hepta­sulfide) were prepared in evacuated fused-silica ampoules via high-temperature, solid-state synthesis using stoichiometric amounts of the elements at 1273 K. These isostructural compounds crystallize in the Cu4NiSi2S7 structure type, which can be considered as a superstructure of cubic diamond or sphalerite. The monovalent (Cu+), divalent (Fe2+ or Co2+) and tetra­valent (Ge4+) cations adopt tetra­hedral geometries, each being surrounded by four S2− anions. The divalent cation and one of the sulfide ions lie on crystallographic twofold axes. These tetra­hedra share corners to create a three-dimensional framework structure. All of the tetra­hedra align along the same crystallographic direction, rendering the structure non-centrosymmetric and polar (space group C2). Analysis of X-ray powder diffraction data revealed that the structures are the major phase of the reaction products. Thermal analysis indicated relatively high melting temperatures, near 1273 K.

The series of quaternary rare-earth phosphides REMnCuP2 (RE = Y, La–Nd, Sm, Gd–Tm, Lu), which had previously been limited to the Yb member, has been prepared by reaction of the elements at 800 °C. These phosphides adopt the CaAl2Si2-type structure (trigonal, space group P 3 ¯ m 1 , Z = 1) with Mn and Cu atoms disordered over the tetrahedral M sites within [M2P2] slabs. However, single-crystal X-ray diffraction studies, carried out on some of these members (RE = Tb, Dy, Er, Tm), provide evidence for an interstitial octahedral site being partially occupied by Cu atoms, resulting in the nonstoichiometric formulation REMnCu1+xP2 (x = 0.10–0.16). A second polymorph of LaMnCuP2 adopting the BaCu2S2-type structure (orthorhombic, space group Pnma, Z = 4) was also identified in the course of these investigations. Electronic structure calculations on a ordered superstructure model of YMnCuP2 indicate the absence of a band gap.

The quaternary rare-earth sulfides Nd7FeInS13 and Pr7CoInS13 were prepared by reactions of the elements at 1050 °C. Single-crystal X-ray diffraction revealed that they adopt a new orthorhombic structure type (space group Pnma, Z =4; a =11.2079(5) A, b =3.9686(2) A, and c =44.1642(18) A for Nd7FeInS13; a =11.2303(6) A, b =3.9876(2) A, and c =44.506(2) A for Pr7CoInS13). The structure contains two types of one-dimensional chains extending along the b-direction: corner-sharing tetrahedra centred by transition-metal atoms (M = Fe, Co), and edge-sharing octahedra centred by In atoms. The RE atoms (RE = Nd, Pr) are coordinated in monocapped or bicapped trigonal prismatic geometry. In terms of these RE-centred trigonal prisms, the structure of Nd7FeInS13 and Pr7CoInS13 can be considered to be built up of fragments of the structures of Y2HfS5 and RE2S3 (β-Cr3C2-type). Magnetic measurements on Pr7CoInS13 indicated paramagnetic behaviour with no transitions down to 2 K.

The four ternary arsenides ATt3As3 (A=K, Rb; Tt=Ge, Sn) were obtained by reaction of the elements at 600–650 °C. They adopt an orthorhombic structure (space group Pnma, Z=4, with cell parameters ranging from a=9.9931(11) A, b=3.7664(4) A, c=18.607(2) A for KGe3As3 to a=10.3211(11) A, b=4.0917(4) A, c=19.570(2) A for RbSn3As3) containing corrugated [Tt3As3] layers built from Tt-centred trigonal pyramids and tetrahedra forming five-membered rings decorated with As handles. They can be considered to be Zintl phases with Tt atoms in +4, +3, and +1 oxidation states. Band structure calculations predict that these compounds are semiconductors with narrow band gaps (0.71 eV in KGe3As3, 0.50 eV in KSn3As3).

The series of quaternary sulfides with general formula Ln2CuMS5 (Ln=La, Ce; M=Sb, Bi) have been synthesized by solid-state reactions. Three representative members have been structurally characterized by single-crystal X-ray diffraction. La2CuSbS5 crystallizes in a new structure type (space group Ima2 (no. 46), Z=4, a=13.401(2) A, b=7.592(1) A, c=7.598(1) A, V=773.1(3) A3). The bismuth analogs of composition La2CuBiS5 and Ce2CuBiS5 crystallize with the La2CuInSe5 structure type (space group Pnma (no. 62), Z=4). Lattice parameters for La2CuBiS5: a=11.9213(5) A, b=3.9967(2) A, c=17.0537(8) A, V=812.56(7) A3; lattice parameters for Ce2CuBiS5: a=11.9179(15) A, b=3.9596(5) A, c=16.955(2) A, V=800.13(17) A3). The similarities and the differences between the two structures are discussed. Electronic structure calculations for La2CuSbS5 and La2CuBiS5 are also presented; they suggest semiconducting behavior with energy gaps exceeding 1.7 eV.

The new ternary arsenides AE3TrAs3 (AE = Sr, Ba; Tr = Al, Ga) and their phosphide analogs Sr3GaP3 and Ba3AlP3 have been prepared by reactions of the respective elements at high temperatures. Single-crystal X-ray diffraction studies reveal that Sr3AlAs3 and Ba3AlAs3 adopt the Ba3AlSb3-type structure (Pearson symbol oC56, space group Cmce, Z = 8). This structure is also realized for Sr3GaP3 and Ba3AlP3. The compounds Sr3GaAs3 and Ba3GaAs3 crystallize with the Ba3GaSb3-type structure (Pearson symbol oP56, space group Pnma, Z = 8). Both structures are made up of isolated pairs of edge-shared AlPn4 and GaPn4 tetrahedra (Pn = pnictogen, i.e., P or As), separated by the alkaline-earth Sr2+ and Ba2+ cations. In both cases, there are no homoatomic bonds, hence, regardless of the slightly different atomic arrangements, both structures can be rationalized as valence-precise [AE2+]3[Tr3+][Pn3−]3, or rather [AE2+]6[Tr2Pn6]12−, i.e., as Zintl phases.

Three series of quaternary rare-earth transition-metal pnictides REMM′Pn2 (M=Cu, Ag; M′=Zn; Pn=P, As) have been prepared by reaction of the elements at 800 °C, with crystal growth promoted through the addition of iodine. The extent of RE substitution is broad in these series: RECuZnAs2 (RE=Y, La Nd, Sm, Gd–Lu), REAgZnP2 (RE=La–Nd, Sm, Gd–Dy), and REAgZnAs2 (RE=La Nd, Sm, Gd Dy). Powder and single-crystal X-ray diffraction analysis revealed that they adopt the trigonal CaAl2Si2-type structure (space group P 3 ¯ m1, Z=1), in which Cu or Ag atoms are disordered with Zn atoms over the unique tetrahedrally coordinated transition-metal site. Magnetic measurements indicated Curie–Weiss behavior for several members of the RECuZnAs2 and REAgZnP2 series. Core-line X-ray photoelectron spectra (XPS) collected on some RECuZnAs2 members corroborate the charge assignment deduced by the Zintl concept for these compounds, (RE3+)(M1+)(Zn2+)(Pn3−)2. Optical diffuse reflectance spectra and valence band XPS spectra established that these compounds are small band-gap semiconductors (up to ~0.8 eV in REAgZnP2) or semimetals (RECuZnAs2). Band structure calculations also support this electronic structure and indicate that the band gap can be narrowed through appropriate chemical substitution (RE=smaller atoms, M=Cu, and Pn=As).

Six series of quaternary rare-earth transition-metal chalcogenides RE3MGaCh7 (M=Fe, Co, Ni; Ch=S, Se), comprising 33 compounds in total, have been prepared by reactions of the elements at 1050 °C (for the sulphides) or 900 °C (for the selenides). They adopt noncentrosymmetric hexagonal structures (ordered Ce3Al1.67S7-type, space group P63, Z=2) with cell parameters in the ranges of a=9.5–10.2 A and c=6.0–6.1 A for the sulphides and a=10.0–10.5 A and c=6.3–6.4 A for the selenides as refined from powder X-ray diffraction data. Single-crystal structures were determined for five members of the sulphide series RE3FeGaS7 (RE=La, Pr, Tb) and RE3CoGaS7 (RE=La, Tb). The highly anisotropic crystal structures consist of one-dimensional chains of M-centred face-sharing octahedra and stacks of Ga-centred tetrahedra all pointing in the same direction. Magnetic measurements on the sulphides reveal paramagnetic behaviour in some cases and long-range antiferromagnetic behaviour with low Neel temperatures (15 K or lower) in others. Ga L-edge XANES spectra support the presence of highly cationic Ga tetrahedral centres with a tendency towards more covalent Ga–Ch character on proceeding from the sulphides to the selenides. Band structure calculations on La3FeGaS7 indicate that the electronic structure is dominated by Fe 3d-based states near the Fermi level.

Six quaternary rare-earth-containing sulphides RE3MInS7 (M=Fe, Co, Ni) have been prepared by reactions of the elements at 1050–1150 °C. They are the first examples of chalcogenides RE3MM'Ch7 adopting related La3CuSiS7- or ordered Ce3Al1.67S7-type structures in which the M' component is indium. They crystallize in the noncentrosymmetric hexagonal space group P63 with Z=2 and cell parameters in the ranges of a=9.95–10.15 A and c=6.25–6.29 A for RE3FeInS7 (RE=La–Pr), RE3CoInS7 (RE=La, Ce), and La3NiInS7. The crystal structure consists of parallel chains of face-sharing M-centred octahedra and stacks of In-centred tetrahedra, all pointing in the same polar direction, with seven-coordinate La atoms in the intervening spaces. X-ray photoelectron spectra support the valence assignments implied by the formulation (RE3+)3(M2+)(In3+)(S2−)7. Magnetic measurements are suggestive of antiferromagnetic coupling between the M moments. Band structure calculations reveal that a band gap of 0.25 eV is present in La3FeInS7, whereas the increased electron count in La3CoInS7 and La3NiInS7 causes the Fermi level to cut narrow bands.

The CaCu4P2-type pnictides EuAg4As2 and EuAg4Sb2 both contain stable divalent europium. Temperature dependent magnetic susceptibility measurements revealed experimental magnetic moments of 7.91(1) and 8.00(1) μB/Eu atom for the arsenide and antimonide, respectively. This is supported by 151Eu Mossbauer spectra which show isomer shifts of −11.33(1) (EuAg4As2) and −11.71(1) mm/s (EuAg4Sb2). Both pnictides show antiferromagnetic ordering below Neel temperatures of 14.9(5) (EuAg4As2) and 11.0(5) K (EuAg4Sb2). The antiferromagnetic ground state is only stable at low field strength. Metamagnetic transitions occur at critical field strengths of 4.4 (arsenide) and 2.4 kOe (antimonide) with saturation near 7 μB at 80 kOe and 5 K, indicating full parallel spin alignment. This is expressed by large magnetic hyperfine fields of 27.6(1) and 26.2(1) T at the europium nuclei of EuAg4As2 and EuAg4Sb2, respectively. 121Sb data of EuAg4Sb2 show a transferred hyperfine field of 9.8(2) T.

Through arc-melting reactions of the elements and annealing at 800 °C, the ternary rare-earth germanides RE 3 Ru 2 Ge 3 and RE 3 Ir 2 Ge 3 have been prepared for most of the smaller RE components ( RE =Y, Gd–Tm, Lu). In the iridium-containing reactions, the new phases RE 2 IrGe 2 were also generally formed as by-products. Powder X-ray diffraction revealed orthorhombic Hf 3 Ni 2 Si 3 -type structures (space group Cmcm , Z =4) for RE 3 M 2 Ge 3 ( M =Ru, Ir) and monoclinic Sc 2 CoSi 2 -type structures (space group C 2/ m , Z =4) for RE 2 IrGe 2 . Full crystal structures were determined by single-crystal X-ray diffraction for all members of RE 3 Ru 2 Ge 3 ( a =4.2477(6) A, b =10.7672(16) A, c =13.894(2) A for RE =Y; a =4.2610(3)–4.2045(8) A, b =10.9103(8)–10.561(2) A, c =14.0263(10)–13.639(3) A in the progression of RE from Gd to Lu) and for Tb 3 Ir 2 Ge 3 ( a =4.2937(3) A, b =10.4868(7) A, c =14.2373(10) A). Both structures can be described in terms of CrB- and ThCr 2 Si 2 -type slabs built from Ge-centred trigonal prisms. However, band structure calculations on Y 3 Ru 2 Ge 3 support an alternative description for RE 3 M 2 Ge 3 based on [ M 2 Ge 3 ] layers built from linked M Ge 4 tetrahedra, which emphasizes the strong M –Ge covalent bonds present. The temperature dependence of the electrical resistivity of RE 3 Ru 2 Ge 3 generally indicates metallic behaviour but with low-temperature transitions visible for some members ( RE =Gd, Tb, Dy) that are probably associated with magnetic ordering of the RE atoms. Anomalously, Y 3 Ru 2 Ge 3 exhibits semiconductor-like behaviour of uncertain origin. Magnetic measurements on Dy 3 Ru 2 Ge 3 reveal antiferromagnetic ordering at 3 K and several unusual field-dependent transitions suggestive of complex spin reorientation processes.

The ternary rare-earth zinc arsenides RE Zn 2 As 3 ( RE =La–Pr) have been prepared by reactions of the elements at 800 °C. They adopt new tetragonal structure types, with the La member (space group I 4/ mmm , Z =4, a =4.1133(2) A, c =32.1246(14) A) differing from the Ce and Pr members (space group P 4/ nmm , Z =2, a =4.0804(6)–4.0534(12) A, c =16.089(5)–16.059(10) A, respectively) in the disposition of RE -centred square antiprisms on opposite sides of square As nets. The structures contain thick anionic slabs, built from edge-sharing ZnAs 4 tetrahedra, that can be considered either as stackings of three PbO-type layers or as fragments of the fluorite-type structure. These slabs are deficient in Zn and are reminiscent of the same features in binary arsenide Zn 3 As 2 . LaZn 2 As 3 exhibits temperature-independent paramagnetism whereas CeZn 2 As 3 follows modified Curie–Weiss behavior consistent with Ce 3+ species but with no occurrence of long-range ordering.

The ternary rare-earth-metal chromium gallides RE 4 CrGa 12 ( RE =Tb–Tm) have been prepared by reactions of the elements at 1000 °C in the presence of excess gallium used as a self-flux. Their structures are derived by inserting Cr atoms into a quarter of the empty Ga 6 octahedral clusters found in the parent binary gallides RE Ga 3 (AuCu 3 -type), although single-crystal X-ray diffraction studies suggest that complex superstructures may be adopted. An ideal ordered Y 4 PdGa 12 -type structure was successfully refined for a crystal of Dy 4 CrGa 12 (Pearson symbol cI 34, space group Im 3 ¯ m , Z =2, a =8.572(1) A). Magnetic measurements on single-crystal samples reveal ferromagnetic or possibly ferrimagnetic ordering for the Tb, Dy, and Er members ( T C =22, 15, and 2.8 K, respectively) and antiferromagnetic ordering for the Ho member ( T N =7.5 K). Band structure calculations on a hypothetical “Y 4 CrGa 12 ” model suggest that the Cr atoms carry no local magnetic moment.

The ternary arsenide BaAg{sub 2}As{sub 2} has been prepared by reaction of the elements at 850 Degree-Sign C. Single-crystal and powder X-ray diffraction analysis revealed that it adopts the ThCr{sub 2}Si{sub 2}-type structure (Pearson symbol tI10, space group I4/mmm, Z=2, a=4.6025(3) A, c=10.8672(6) A at 295 K) featuring [Ag{sub 2}As{sub 2}] layers interconnected by homoatomic As-As bonds along the c-direction. Band structure calculations indicate no gap at the Fermi level, and support the occurrence of strong As-As and weak Ag-Ag bonding. The asymmetric lineshape and the absence of a BE shift in the Ag 3d{sub 5/2} core-line peak relative to the element suggest delocalization of the Ag valence electrons. A significant negative BE shift (1.0 eV) in the As 3d{sub 5/2} core-line peak relative to the element confirms the presence of anionic As atoms. A reversible transition is observed at 175 K in the electrical resistivity, and is probably related to a structural phase transition. - Graphical abstract: BaAg{sub 2}As{sub 2} adopts a ThCr{sub 2}Si{sub 2}-type structure with As-As pairs and undergoes a transition at 175 K in its electrical resistivity. Highlights: Black-Right-Pointing-Pointer BaAg{sub 2}As{sub 2} fills the gap within the BaM{sub 2}As{sub 2} series containing 4d metals. Black-Right-Pointing-Pointer [Ag{submore » 2}As{sub 2}] layers are linked by As-As pairs. Black-Right-Pointing-Pointer XPS studies support the assignment of monovalent Ag and anionic As atoms. Black-Right-Pointing-Pointer An anomaly is observed at 175 K in the electrical resistivity.« less

Four ternary pnictides AAg{sub 4}Pn{sub 2} (A=Sr, Eu; Pn=As, Sb) were prepared by reactions of the elements at 850 Degree-Sign C and their crystal structures were determined from single-crystal X-ray diffraction studies. These silver-containing pnictides AAg{sub 4}Pn{sub 2} adopt the trigonal CaCu{sub 4}P{sub 2}-type structure (Pearson symbol hR21, space group R3-bar m, Z=3; a=4.5555(6) A, c=24.041(3) A for SrAg{sub 4}As{sub 2}; a=4.5352(2) A, c=23.7221(11) A for EuAg{sub 4}As{sub 2}; a=4.7404(4) A, c=25.029(2) A for SrAg{sub 4}Sb{sub 2}; a=4.7239(3) A, c=24.689(2) A for EuAg{sub 4}Sb{sub 2}), which can be derived from the trigonal CaAl{sub 2}Si{sub 2}-type structure of the isoelectronic zinc-containing pnictides AZn{sub 2}Pn{sub 2} by insertion of additional Ag atoms into trigonal planar sites within [M{sub 2}Pn{sub 2}]{sup 2-} slabs built up of edge-sharing tetrahedra. Band structure calculations on SrAg{sub 4}As{sub 2} and SrAg{sub 4}Sb{sub 2} revealed that these charge-balanced Zintl phases actually exhibit no gap at the Fermi level and are predicted to be semimetals. - Graphical abstract: SrAg{sub 4}As{sub 2} and related pnictides adopt a CaCu{sub 4}P{sub 2}-type structure in which additional Ag atoms enter trigonal planar sites within slabs built from edge-sharing tetrahedra. Highlights: Black-Right-Pointing-Pointer AAg{sub 4}Pn{sub 2} are the first Ag-containing members of the CaCu{sub 4}P{submore » 2}-type structure. Black-Right-Pointing-Pointer Ag atoms are stuffed in trigonal planar sites within CaAl{sub 2}Si{sub 2}-type slabs. Black-Right-Pointing-Pointer Ag-Ag bonding develops through attractive d{sup 10}-d{sup 10} interactions.« less

The ternary rare-earth zinc arsenides RE Zn 1− x As 2 ( RE =La–Nd, Sm) were prepared by reaction of the elements at 800 °C. Single-crystal and powder X-ray diffraction analysis revealed a defect SrZnBi 2 -type average structure for the La member (Pearson symbol tI 16, space group I 4/ mmm , Z =4; a =4.0770(9) A, c =20.533(5) A), in contrast to defect HfCuSi 2 -type average structures for the remaining RE members (Pearson symbol tP 8, space group P 4/ nmm , Z =2; a =4.0298(5)–3.9520(4) A, c =10.222(1)–10.099(1) A in the progression from Ce to Sm). The homogeneity range is not appreciable (estimated to be narrower than 0.6 x 1− x As 2 ) and the formula RE Zn 0.67 As 2 likely represents the Zn-rich phase boundary. The Ce–Nd members are Curie–Weiss paramagnets. LaZn 0.67 As 2 shows activated behavior in its electrical resistivity, whereas SmZn 0.67 As 2 exhibits anomalies in its temperature dependence of the electrical resistivity.

The quaternary rare-earth phosphides RECuZnP2 (RE=Pr, Nd, Gd–Tm, Lu) have been prepared by reaction of the elements at 900 °C, completing this versatile series which forms for nearly all RE metals. They adopt the trigonal CaAl2Si2-type structure (Pearson symbol hP5, space group P3m1, Z=1), as confirmed by single-crystal X-ray diffraction analysis on ErCuZnP2 and powder X-ray diffraction analysis on the remaining members. The Cu and Zn atoms are assumed to be disordered over the single transition-metal site. Band structure calculations on a hypothetically ordered YCuZnP2 model suggest a semimetal, with a zero band gap between the valence and conduction bands. This electronic structure is supported by XPS valence band spectra for RECuZnP2 (RE=Gd–Er), in which the intensity drops off smoothly at the Fermi edge. The absence of a band gap permits the electron count to deviate from the precise value of 16 e− per formula unit, as demonstrated by the formation of a solid solution in GdCuxZn2−xP2 (1.0≤x≤1.3), while still retaining the CaAl2Si2-type structure. Because the Cu 2p XPS spectra indicate that the Cu atoms are always monovalent, the substitution of Cu for Zn leads to a decrease in electron count and a lowering of the Fermi level in the valence band. The magnetic susceptibility of RECuZnP2 (RE=Gd–Er), which obeys the Curie–Weiss law, confirms the presence of trivalent RE atoms.

which has a superconducting transition temperature of 2.3 K, provides an excellent opportunity tostudytheinterplayofsuperconductivityandmagnetismnearanantiferromagneticquantumcriticalpoint.Previousstudies have explored the effects of electron doping, magnetic field, and positive pressure on this system. Todetermine the effect of hole doping, we have grown single crystals of Ce(Co