Crystal structures and magnetic properties of two-dimensional antiferromagnets Co1−xZnxTeMoO6

Crystal structures and magnetic properties of metal telluromolybdates Co_1−xZn_xTeMoO₆ (x=0.0, 0.1,…,0.9) are reported. All the compounds have an orthorhombic structure with space group P2₁2₁2 and a charge configuration of M²⁺Te⁴⁺Mo⁶⁺O₆. In this structure, M ions form a pseudo-two-dimensional lattice in the ab plane. Their magnetic susceptibility measurements have been performed in the temperature range between 1.8 and 300 K. The end member CoTeMoO₆ shows a magnetic transition at 24.4 K. The transition temperature for solid solutions rapidly decreases with increasing x and this transition disappears between x=0.4 and 0.5, which is corresponding to the percolation limit for the square-planer lattice. From the magnetization, specific heat, and powder neutron diffraction measurements, it is found that the magnetic transition observed in the CoTeMoO₆ is a canted antiferromagnetic ordering of Co²⁺ ions. The antiferromagnetic component of the ordered magnetic moment (3.12(3)μ_B at 10 K) is along the b-axis. In addition, there exists a small ferromagnetic component (0.28(3)μ_B) along the a-axis.     

Magnetic properties of EuLn2O4 (Ln=rare earths)

Ternary rare earth oxides EuLn₂O₄ (Ln=Gd, Dy-Lu) were prepared. They crystallized in an orthorhombic CaFe₂O₄-type structure with space group Pnma. ¹⁵¹Eu Mössbauer spectroscopic measurements show that the Eu ions are in the divalent state. All these compounds show an antiferromagnetic transition at 4.2-6.3 K. From the positive Weiss constant and the saturation of magnetization for EuLu₂O₄, it is considered that ferromagnetic chains of Eu²⁺ are aligned along the b-axis of the orthorhombic unit cell, with neighboring Eu²⁺ chains antiparallel. When Ln=Gd-Tm, ferromagnetically aligned Eu²⁺ ions interact with the Ln³⁺ ions, which would overcome the magnetic frustration of triangularly aligned Ln³⁺ ions and the EuLn₂O₄ compounds show a simple antiferromagnetic behavior.     

Crystal structure and electrical conductivity of lanthanum-calcium chromites-titanates La1−xCaxCr1−yTiyO3−δ (x=0-1, y=0-1)

Five series of perovskite-type compounds in the system La_1−xCa_xCr_1−yTi_yO₃ with the nominal compositions y=0, x=0-0.5; y=0.2, x=0.2-0.8; y=0.5, x=0.5-1.0; y=0.8, x=0.6-1.0 and y=1, x=0.8-1 were synthesized by a ceramic technique in air (final heating 1350 °C). On the basis of the X-ray analysis of the samples with (Ca/Ti)?1, the phase diagram of the CaTiO₃-LaCr^IIIO₃-CaCr^IVO₃ quasi-ternary system was constructed. Extended solid solution with a wide homogeneity range is formed in the quasi-ternary system CaCr^IVO₃-CaTiO₃-LaCr^IIIO₃. The solid solution La_{(1−x′−y)}Ca_(x′+y)Cr^IV_x′Cr^III_{(1−x′−y)}Ti_yO₃ exists by up to 0.6-0.7 mol fractions of CaCr^IVO₃ (x^′<0.6-0.7) at the experimental conditions. The crystal structure of the compounds is orthorhombic in the space group Pbnm at room temperature. The lattice parameters and the average interatomic distances of the samples within the solid solution ranges decrease uniformly with increasing Ca content. Outside the quasi-ternary system, the nominal compositions La_0.1Ca_0.9TiO₃, La_0.2Ca_0.8TiO₃, La_0.4Ca_0.6Cr_0.2Ti_0.8O₃ and La_0.3Ca_0.7Cr_0.2Ti_0.8O₃ in the system La_1−xCa_xCr_1−yTi_yO₃ were found as single phases with an orthorhombic structure. In the temperature range between 850 and 1000 °C, the synthesized single-phase compositions are stable at pO₂=6×10⁻¹⁶-0.21×10⁵ Pa. Oxygen stoichiometry and electrical conductivity of the separate compounds were investigated as functions of temperature and oxygen partial pressure. The chemical stability of these oxides with respect to oxygen release during thermal dissociation decreases with increasing Ca-content. At 900 °C and oxygen partial pressure 1×10⁻¹⁵-0.21×10⁵ Pa, the compounds with x>y (acceptor doped) are p-type semiconductors and those with x<y (donor doped) and x=y are n-type semiconductors. The type and level of electrical conductivity are functions of the concentration ratios of cations occupying the B-sites of the perovskite structures: [Cr³⁺]/[Cr⁴⁺] and [Ti⁴⁺]/[Ti³⁺]. The maximum electrical conductivity at 900 °C and pO₂=10⁻¹⁵ Pa was found for the composition La_0.1Ca_0.9TiO₃ (near 50 S/cm) and in air at 900 °C for La_0.5Ca_0.5CrO₃ (close to 100 S/cm).     

Structural and magnetic study of the cation-ordered perovskites Ba2−xSrxErMoO6

A series of perovskite phases have been prepared from the appropriate carbonates and oxides by heating under reducing conditions at temperatures up to 1300 °C. Complete ordering between ErO₆ and MoO₆ octahedra and a disordered distribution of Sr²⁺ and Ba²⁺ occur in all compounds. Neutron powder diffraction experiments show that the substitution of Sr²⁺ into Ba₂ErMoO₆ introduces a progressive reduction in symmetry from Fm3¯m (x=0) to I4/m (x=0.5, 0.8) to P2₁/n (x=1.25, 1.75, 2.0). Magnetic susceptibility measurements indicate that all of these compounds show Curie-Weiss paramagnetism and that for x<1.25 this behaviour persists down to 2 K. The monoclinically distorted compounds show magnetic transitions at low temperature and neutron diffraction has confirmed the presence of long-range antiferromagnetic order below 2.5 and 4 K in Ba_0.25Sr_1.75ErMoO₆ and Sr₂ErMoO₆, respectively. Ba_0.75Sr_1.25ErMoO₆, Ba_0.25Sr_1.75ErMoO₆ and Sr₂ErMoO₆ do not undergo structural distortion on cooling from room temperature.     

The novel silicide Eu3Si4: structure, chemical bonding, magnetic behavior and electrical resistivity

The novel binary europium silicide Eu₃Si₄ was synthesized from the elements. Its crystal structure is a derivative of the Ta₃B₄ type: space group Immm, a=4.6164(4) Å, b=3.9583(3) Å, c=18.229(1) Å, Z=2. In the structure, the silicon atoms form one-dimensional bands of condensed hexagons. Deviating from the prototype structure, a partial corrugation of the initially planar bands may be concluded from the analysis of the experimental electron density in the vicinity of the Si1 atoms. In the paramagnetic region, Eu₃Si₄ shows a 4f⁷ electronic configuration for the europium atoms. Two consecutive magnetic ordering transitions were found at 117 and 40 K. The first one is attributed to a ferromagnetic ordering of the Eu2 atoms; the second one is caused by a ferromagnetic ordering of the Eu1 atoms resulting in a ferrimagnetic ground state with a net magnetization of 7 μ_B at 1.8 K. The temperature dependence of the electrical resistivity reflects the metallic character of the investigated compound. Furthermore, the pronounced changes of the dρ/dT slope confirm the magnetic transitions. From bonding analysis with the electron localization function, Eu₃Si₄ shows a Zintl-like character and its electronic count balance can be written as (Eu^1.83+)₃(Si1^0.95−)₂(Si2^1.8−)₂, in good agreement with its magnetic behavior in the paramagnetic region.     

Electron doping effect on structural and magnetic phase transitions in Sr2−xNdxFeMoO6 double perovskites

Polycrystalline Sr_2−xNd_xFeMoO₆ (x=0.0, 0.1, 0.2, 0.4) materials have been synthesized by a citrate co-precipitation method and studied by neutron powder diffraction (NPD) and magnetization measurements. Rietveld analysis of the temperature-dependent NPD data shows that the compounds (x=0.0, 0.1, 0.2) crystallize in the tetragonal symmetry in the range 10-400 K and converts to cubic symmetry above 450 K. The unit cell volume increases with increasing Nd³⁺ concentration, which is an electronic effect in order to change the valence state of the B-site cations. Antisite defects at the Fe-Mo sublattice increases with the Nd³⁺ doping. The Curie temperature was increased from 430 K for x=0 to 443 K for x=0.4. The magnetic moment of the Fe-site decreases while the Mo-site moment increases with electron doping. The antiferromagnetic arrangement causes the system to show a net ferrimagnetic moment.     

Enhancement of magnetic ordering temperature in iron substituted ytterbium manganate (YbMn1−xFexO3)

Oxides of the type YbMn_1−xFe_xO₃; x≤0.3 showing multiferroic behavior have been synthesized by the solid state route. These oxides crystallize in the hexagonal structure known for the parent YbMnO₃ with the c/a ratio increasing with Fe substitution. The distortion of the MnO₅ polyhedra (tbp) decreases and the Mn-O-Mn bonds in the a-b plane become shorter with Fe-substitution. Magnetic ordering is observed from the low temperature neutron diffraction study. The compounds were found to be antiferromagnetic and the ordering temperature T_N increased from 82 K for pure YbMnO₃ to 95 K for YbMn_0.7Fe_0.3O₃. Variable temperature dielectric measurements (15-110 K) show an anomaly in the dielectric constant at temperatures close to the antiferromagnetic ordering temperature for all the compositions, showing a unique correlation between the magnetic and electric field. The increase in the ordering temperature in YbMn_1−xFe_xO₃ is explained on the basis of increase in covalence of Mn/Fe-O-Mn/Fe bonds (shorter) with iron substitution.     

Preparation, crystal structure, and photoluminescence of Ca2SnO4:Eu, Y

Eu³⁺-doped Ca₂SnO₄ (solid solutions of Ca_2−xEu_2xSn_1−xO₄, 0?x?0.3) and Eu³⁺ and Y³⁺-codoped Ca₂SnO₄ (Ca_1.8Y_0.2Eu_0.2Sn_0.8O₄) were prepared by solid-state reaction at 1400 °C in air. Rietveld analysis of the X-ray powder diffraction patterns revealed that Eu³⁺ replaced Ca²⁺ and Sn⁴⁺ in Eu³⁺-doped Ca₂SnO₄, and that Eu³⁺ replaced Ca²⁺ and Y³⁺ replaced Sn⁴⁺ in Ca_1.8Y_0.2Eu_0.2Sn_0.8O₄. Red luminescence at 616 nm due to the electric dipole transition ⁵D_o→⁷F₂ was observed in the photoluminescence (PL) spectra of Ca_2−xEu_2xSn_1−xO₄ and Ca_1.8Y_0.2Eu_0.2Sn_0.8O₄ at room temperature. The maximum PL intensity in the solid solutions of Ca_2−xEu_2xSn_1−xO₄ was obtained for x=0.1. The PL intensity of Ca_1.8Y_0.2Eu_0.2Sn_0.8O₄ was 1.26 times greater than that of Ca_2−xEu_2xSn_1−xO₄ with x=0.1.     

Magnetic properties and Eu Mössbauer effects of mixed valence europium copper sulfide, Eu2CuS3

Ternary europium copper sulfide Eu₂CuS₃ have been investigated by X-ray diffraction, ¹⁵¹Eu Mössbauer spectroscopy, magnetic susceptibility, magnetization, and specific heat measurements. In this compound, Eu²⁺ and Eu³⁺ ions occupy two crystallographically independent sites. The ¹⁵¹Eu Mössbauer spectra indicate that the Eu²⁺ and Eu³⁺ ions exist in the molar ratio of 1:1, and the Debye temperatures of Eu²⁺ and Eu³⁺ are 180 and 220 K, respectively. In its magnetic susceptibility, the divergence between the zero-field cooled and field cooled susceptibilities appears below 3.4 K. The specific heat has a λ-type anomaly at the same temperature. From the field dependence of magnetization at 1.8 K, the Eu²⁺ ion was found to be in the ferromagnetic state with the saturation magnetization M_S=6.7 μ_B.     

Thermoelectric properties of polycrystalline La1−xSrxCoO3

Thermoelectric properties of polycrystalline La_1−xSr_xCoO₃, where Sr²⁺ is substituted in La³⁺ site in perovskite-type LaCoO₃, have been investigated. Sr-doping increases the electrical conductivity (σ) of La_1−xSr_xCoO₃, and also decreases the Seebeck coefficient (S) for 0.01?x?0.40. A Hall coefficient measurement reveals that the increase in electrical conductivity arises from increases in both carrier concentration and the Hall mobility. The decrease in the Seebeck coefficient is caused by a decrease in carrier effective mass as well as increase in carrier concentration. The highest power factor (σS²) is 3.7×10⁻⁴ W m⁻¹ K⁻² at 250 K for x=0.10. The thermal conductivity (κ) is about 2 W m⁻¹ K⁻¹ at 300 K for 0?x?0.04, and increases for x?0.05 because of an increase in heat transport by conductive carrier. The thermoelectric properties of La_1−xSr_xCoO₃ are improved by Sr-doping, and the figure of merit (Z=σS² κ⁻¹) reaches 1.6×10⁻⁴ K⁻¹ for x=0.06 at 300 K (ZT=0.048). For heavily Sr-doped samples, the thermoelectric properties diminish mainly because of the decrease in the Seebeck coefficient and the increase in thermal conductivity.     

Novel powellite-based red-emitting phosphors: CaLa1−xNbMoO8:xEu for white light emitting diodes

We report the photoluminescence properties of a novel powellite-based red-emitting phosphor material: CaLa_1−xNbMoO₈:xEu³⁺ (0.01, 0.03, 0.05, 0.1) for the first time. The photoluminescence investigations indicated that CaLa_1−xNbMoO₈:xEu³⁺ emits strong red light at 615 nm originating from ⁵D₀→⁷F₂ (electric dipole transition) under excitation either into the ⁵L₀ state with 394 nm or the ⁵D₂ state with 464 nm, that correspond to the two popular emission lines from near-UV and blue LED chips, respectively. When compared with emission intensity from a CaMoO₄:Eu³⁺, the emission from CaLaNbMoO₈:Eu³⁺ showed greater intensity values under the same excitation wavelength (394 nm). The enhanced red emission is attributed to the enhanced f-f absorption of Eu³⁺. These materials could be promising red phosphors for use in generating white light in phosphor-converted white light emitting diodes (WLEDs).     

Preparation and photoluminescence property of a loose powder, Ca3Al2O6:Eu by calcination of a layered double hydroxide precursor

A novel red light-emitting material, Ca₃Al₂O₆:Eu³⁺, which is the first example found in the Ca₃Al₂O₆ host, was prepared by calcination of a layered double hydroxide precursor at 1350 °C. The precursor, [Ca_2.9−xAl₂Eu_x(OH)_9.8](NO₃)_2+x·2.5H₂O, was prepared by coprecipitation of metal nitrates with sodium hydroxide. The material is a loose powder composed of irregular particles formed from aggregation of particles of a few nanometers, as shown in scanning electron microscope (SEM) images. It was found that the photoluminescence intensity reached the maximum when the calcination temperature was 1350 °C and the concentration of Eu³⁺ was 1.0%. The material emits bright red emission at 614 nm under a radiation of λ=250 nm.     

Crystal and magnetic structures of the brownmillerite compound Ca2Fe1.039(8)Mn0.962(8)O5

The crystal and magnetic structures of the brownmillerite material, Ca₂Fe_1.039(8)Mn_0.962(8)O₅ were investigated using powder X-ray and neutron diffraction methods, the latter from 3.8 to 700 K. The compound crystallizes in Pnma space group with unit cell parameters of a=5.3055(5) Å, b=15.322(2) Å, c=5.4587(6) Å at 300 K. The neutron diffraction study revealed the occupancies of Fe³⁺ and Mn³⁺ ions in both octahedral and tetrahedral sites and showed some intersite mixing and a small, ∼4%, Fe excess. While bulk magnetization data were inconclusive, variable temperature neutron diffraction measurements showed the magnetic transition temperature to be 407(2) K below which a long range antiferromagnetic ordering of spins occurs with ordering wave vector k=(000). The spins of each ion are coupled antiferromagnetically with the nearest neighbors within the same layer and coupled antiparallel to the closest ions from the neighboring layer. This combination of intra- and inter-layer antiparallel arrangement of spins forms a G-type magnetic structure. The ordered moments on the octahedral and tetrahedral sites at 3.8 K are 3.64(16) and 4.23(16) μ_B, respectively.     

Preparation, structure and photoluminescence properties of Eu and Ce-doped SrYSi4N7

Undoped and Eu²⁺ or Ce³⁺-doped SrYSi₄N₇ were synthesized by solid-state reaction method at 1400-1660 °C under nitrogen/hydrogen atmosphere. The crystal structure was refined from the X-ray powder diffraction data by the Rietveld method. SrYSi₄N₇ and EuYSi₄N₇, being isotypic with the family of compounds MYbSi₄N₇ (M=Sr, Eu, Ba) and BaYSi₄N_7, crystallize with the hexagonal symmetry: space group P6₃mc (No. 186), Z=2, a=6.0160 (1) Å, c=9.7894 (1) Å, V=306.83(3) Å³; and a=6.0123 (1) Å, c=9.7869 (1) Å, V=306.37(1) Å³, respectively. Photoluminescence properties have been studied for Sr_1−xEu_xYSi₄N₇ (x=0-1) and SrY_1−xCe_xSi₄N₇ (x=0-0.03) at room temperature. Eu²⁺-doped SrYSi₄N₇ shows a broad yellow emission band peaking around 548-570 nm, while Ce³⁺-doped SrYSi₄N₇ exhibits a blue emission band with a maximum at about 450 nm. SrYSi₄N₇:Eu²⁺ can be very well excited by 390 nm radiation, which makes this material attractive as conversion phosphor for LED lighting applications.     

Crystal structures and phase transition in the system SrTiO3-La2/3TiO3

The distribution of A-site cations in the perovskite system La_xSr_1−3x/2TiO₃ depends on the concentration of La³⁺ ions and associated vacancies. For small x (x?0.2), the substitutions are expected to be random. For x?0.55, the cations are ordered in such a way that successive layers of A-sites are occupied to greater and lesser degree, and this ordering drives a tetragonal distortion. For x from about 0.3 to 0.5, the X-ray patterns show diffuse peaks indicative of similar ordering, but this is not long-range order and no tetragonal distortion results. The lower temperature structures also exhibit out-of-phase tilting of the TiO₆ octahedra, setting in at temperatures varying linearly with composition from 105 K for x=0, to about 650 K at x=2/3.     

The binary gallide Eu5Ga9 - crystal structure, chemical bonding and physical properties

The title compound was prepared from the elements by reaction in a sealed tantalum tube at 1320 K followed by slow cooling to 970 K or, alternatively, in glassy carbon crucibles with HF melting. The crystal structure of Eu₅Ga₉ was refined from single-crystal data: Cmcm, a=4.613(1) Å, b=10.902(3) Å, c=26.097(6) Å, Z=4, R_F=0.036, 811 structure factors and 46 variables. The structure is described as a three-dimensional network formed by gallium atoms with europium atoms embedded in the cavities. The bonding analysis (LMTO, ELF) confirmed this representation of the structure. Magnetic susceptibility measurements show Curie-Weiss behavior above 60 K with a magnetic moment per Eu atom of 8.12(1) μ_B, indicating divalent europium. Eu₅Ga₉ orders antiferromagnetically at 19.0(5) K with re-ordering at 6.0(5) K. The electrical resistivity shows a metallic temperature dependence and magnetic scattering. ¹⁵¹Eu Mössbauer spectroscopic experiments are compatible with divalent europium and show complex magnetic hyperfine field splitting below the ordering temperature.     

Eu7Ga6Sb8: A Zintl phase with Ga-Ga bonds and polymeric gallium antimonide chains

The Zintl phase Eu₇Ga₆Sb₈ was obtained from a direct element combination reaction at 900°C. It crystallizes in the orthorhombic space group Pbca (No. 61) with a=15.6470(17) Å, b=17.2876(19) Å, c=17.9200(19) Å, and Z=8. In Eu₇Ga₆Sb₈, the anionic framework forms infinite chains of [Ga₆Sb₈]¹⁴⁻ which are arranged side by side to make a sheet-like arrangement but without linking. The sheets of chains are separated by Eu²⁺ atoms and also within the sheet, Eu²⁺ atoms fill the spaces between two chains. The chain is made up of homoatomic tetramers (Ga₄)⁶⁺ and dimers (Ga₂)⁴⁺ connected by Sb atoms. The compound is a narrow band-gap semiconductor with E_g∼0.6 eV and satisfies the classical Zintl concept. Extended Hückel band structure calculations confirm that the material is a semiconductor and suggest that the structure is stabilized by strong Ga-Ga covalent bonding interactions. Magnetic susceptibility measurements for Eu₇Ga₆Sb₈ show that the Eu atoms are divalent and the compound has an antiferromagnetic transition at 9 K.     

Specific heat and neutron diffraction study on quaternary sulfides BaNd2CoS5 and BaNd2ZnS5

Magnetic and electrical properties are investigated for quaternary neodymium sulfides BaNd₂TS₅ (T=Co, Zn) through the specific heat, neutron diffraction, and electrical conductivity measurements. Their electrical conductivities show semiconductive behavior, which follows the Arrhenius temperature dependence with the activation energy of E_a=1.46 eV for BaNd₂ZnS₅ and E_a=1.19 eV for BaNd₂CoS₅. The specific heat of BaNd₂ZnS₅ has a λ-type anomaly at 2.8 K due to the antiferromagnetic ordering of the Nd³⁺ moments and a Schottky-type anomaly at around 60 K, which results from the crystal field splitting of the ⁴I_9/2 ground state of the Nd³⁺ ion. The specific heat of BaNd₂CoS₅ shows two λ-type anomalies at 5.7 K due to the antiferromagnetic ordering of Nd³⁺ and at 58 K due to the antiferromagnetic ordering of Co²⁺. The latter overlaps with the Schottky-type anomaly due to the crystal field splitting of the Nd³⁺ ion. Neutron diffraction measurements for BaNd₂CoS₅ show that a magnetic arrangement of the Co²⁺ moments has a collinear antiferromagnetic structure, while that of the Nd³⁺ moments has a noncollinear one.     

Synthesis, structure, and magnetic properties of SrMn1−xGaxO3−δ (x=0-0.5) perovskites

We report the synthesis of SrMn_1−xGa_xO_3−δ perovskite compounds and describe the dependence of their phase stability and structural and physical properties over extended cation and oxygen composition ranges. Using special synthesis techniques derived from thermogravimetric measurements, we have extended the solubility limit of random substitution of Ga³⁺ for Mn in the cubic perovskite phase to x=0.5. In the cubic perovskite phase the maximum oxygen content is close to 3−x/2, which corresponds to 100% Mn⁴⁺. Maximally oxygenated solid solution compounds are found to order antiferromagnetically for x=0-0.4, with the transition temperature linearly decreasing as Ga content increases. Increasing the Ga content introduces frustration into the magnetic system and a spin-glass state is observed for SrMn_0.5Ga_0.5O_2.67(3) below 12 K. These properties are markedly different from the long-range antiferromagnetic order below 180 K observed for the layer-ordered compound Sr₂MnGaO_5.50 with nominally identical chemical composition.     

Magnetostructural correlations in the antiferromagnetic Co2−x Cux(OH)AsO4 (x=0 and 0.3) phases

The Co_2−xCu_x(OH)AsO₄ (x=0 and 0.3) compounds have been synthesized under mild hydrothermal conditions and characterized by X-ray single-crystal diffraction and spectroscopic data. The hydroxi-arsenate phases crystallize in the Pnnm orthorhombic space group with Z=4 and the unit-cell parameters are a=8.277(2) Å, b=8.559(2) Å, c=6.039(1) Å and a=8.316(1) Å, b=8.523(2) Å, c=6.047(1) Å for x=0 and 0.3, respectively. The crystal structure consists of a three-dimensional framework in which M(1)O₅-trigonal bipyramid dimers and M(2)O₆-octahedral chains (M=Co and Cu) are present. Co₂(OH)AsO₄ shows an anomalous three-dimensional antiferromagnetic ordering influenced by the magnetic field below 21 K within the presence of a ferromagnetic component below the ordering temperature. When Co²⁺ is partially substituted by Cu²⁺ions, Co_1.7Cu_0.3(OH)AsO₄, the ferromagnetic component observed in Co₂(OH)AsO₄ disappears and the antiferromagnetic order is maintained in the entire temperature range. Heat capacity measurements show an unusual magnetic field dependence of the antiferromagnetic transitions. This λ-type anomaly associated to the three-dimensional antiferromagnetic ordering grows with the magnetic field and becomes better defined as observed in the non-substituted phase. These results are attributed to the presence of the unpaired electron in the dx²−y² orbital and the absence of overlap between neighbour ions.