Neutron diffraction investigation of the evolution of the crystal structure of oxygen-conducting solid solutions (Yb1 ? x Ca x )2Ti2O7 ( x = 0, 0.05, 0.10)

The crystal structure of pyrochlore-like solid solutions (Yb_{1 − x} Ca _x )₂Ti₂O₇ (x = 0, 0.05, 0.10) synthesized by high-temperature annealing of mechanically activated initial oxides at temperatures of 1300–1500°C is studied using neutron diffraction. It is found that the Ca²⁺ cations are located in the ytterbium sublattice, which apparently favors the splitting of one of the oxygen sublattices [O(2) (48f)] of the pyrochlore structure: the decrease in the occupancy of this sublattice is accompanied by the formation of a new sublattice O(3) (8b), whereas the other oxygen sublattice O(1) (8a) remains unchanged. This rearrangement of the anions in the oxygen subsystem due to the incorporation of an alkaline-earth cation explains the high ionic conductance (∼0.2 S/cm at 1000°C) for (Yb_0.9Ca_0.1)₂Ti₂O₇ ∼ 0.2, which is the maximum value observed to date for pyrochlores of the A ₂ B ₂O₇ type, where A = Ln and B is a Subgroup IVA element of the periodic system. The bulk and grain-boundary components of the conductivity of (Yb_0.95Ca_0.05)₂Ti₂O₇ synthesized at temperatures of 1300, 1400, and 1500°C are studied using impedance spectroscopy. It is found that the (Yb_0.95Ca_0.05)₂Ti₂O₇ sample synthesized at 1500°C has the highest total conductivity due to the increased grain-boundary component. The bulk component of the ionic conductivity of (Yb_0.95Ca_0.05)₂Ti₂O₇ is hardly affected by the synthesis temperature and depends mainly on the degree of heterovalent substitution. Original Russian Text ? A.V. Shlyakhtina, A.E. Sokolov, V.A. Ul’yanov, V.A. Trunov, M.V. Boguslavskiĭ, A.V. Levchenko, L.G. Shcherbakova, 2009, published in Kristallografiya, 2009, Vol. 54, No. 1, pp. 31–36.     

Growth and study of single crystals of Pb1 − x BaxSc0.5Nb0.5O3 solid solutions

Crystals of Pb_{1 ? x} Ba_xSc_0.5Nb_0.5O₃ solid solutions with 0 ≤ x ≤ 0.58 have been grown by the method of mass crystallization from flux. It is established that, unlike the concentration dependence of the corresponding ceramic, the concentration dependence of the temperature T _m (the maximum dielectric constant ε in crystals) does not attain saturation. Cooling of crystals with x ≤ 0.04 resulted in a spontaneous transition from the relaxor to the macrodomain ferroelectric state. In crystals with a higher barium content, the relaxor state is “locked in.”     

Growth and characterization of near-band-edge transitions in β-In2S3 single crystals

Single crystals of β-In₂S₃ were grown by chemical vapor transport method using ICl₃ as a transport agent. The below and above band-edge transitions in β-In₂S₃ have been characterized using optical absorption and photoluminescence (PL) measurements in the temperature range 20–300 K. Thermoreflectance (TR) and photoconductivity (PC) measurements were carried out to verify the band-edge nature of the diindium trisulfide tetragonal crystals. Experimental analyses of the transmittance, PL, PC, and TR spectra of β-In₂S₃ confirmed that the chalcogenide compound is a direct semiconductor with a band gap of about 1.935 eV at 300 K. β-In₂S₃ is familiar with its defect nature. For the β-In₂S₃ crystals, two defect emissions and two above band-edge luminescences were simultaneously detected in the PL spectra at low temperatures. The energy variations of the defect emissions showed temperature-insensitive behavior with respect to the temperature change from 20 to 300 K. Temperature dependences of transition energies of the near-band-edge (NBE) transitions below and above band gap are analyzed. The origins for the NBE transitions in the β-In₂S₃ defect crystals are discussed.     

Defect structure and ionic conductivity of Ca1 ? xScxF2 + x (0.02 ≤ x ≤ 0.15) single crystals

Single crystals of the Ca_{1 − x} Sc _x F_{2 + x} (x = 0.106, 0.132, 0.156) solid solutions (CaF₂ structure type, space group Fm m) are investigated using X-ray diffraction. It is revealed that the crystals under investigation contain vacancies in the 8c positions and interstitial fluorine ions in the 48i positions. The coordination number of Sc³⁺ ions in the structure of the Ca_{1 −x} Sc _x F_{2 + x} solid solutions is equal to eight. The specific features of the concentration dependences of the ionic conductivity and the activation energy of ion transfer for the Ca_{1 − x} Sc _x F_{2 + x} (0.02 ≤ x ≤ 0.15) solid solutions are explained in the framework of the percolation model of conducting “defect regions.” The percolation threshold equal to 3–5 mol % ScF₃ corresponds to the model of [Ca_{14 − n} Sc _n F₆₈] octacubic clusters containing fluorine ions in the 48i positions. The ionic conductivity of the Ca_{1 − x} Sc _x F_{2 + x} solid solutions is analyzed in comparison with the change in this characteristic for the series of Ca_0.8 R _0.2F_2.2 crystals with rare-earth elements. Original Russian Text ? E.A. Sulyanova, V.N. Molchanov, N.I. Sorokin, D.N. Karimov, S.N. Sulyanov, B.P. Sobolev, 2009, published in Kristallografiya, 2009, Vol. 54, No. 4, pp. 612–622.     

Crystal and magnetic structures of the Bi1 ? xSrxFeO3 ? δ and Bi1 ? xAxFe1 ? xMnxO3 (A = Sr, Ca) solid solutions

Bi_{1 − x} Sr _x FeO_{3 − x/2} (I), Bi_{1 − x} Sr _x Fe_{1 − x} MnxO₃ (II), and Bi_{1 − x} Ca _x Fe_{1 − x} Mn _x O₃ (III) solid solutions have been obtained. Their magnetization has been measured by X-ray and neutron diffraction and M?ssbauer spectroscopy. According to the M?ssbauer spectroscopy data, iron ions are in the trivalent state in system I. Near the concentration x ≈ 0.2, rhombohedral distortions (sp. gr. R3c) are transformed into tetragonal (P4/mmm). The symmetry of system II changes at x > 0.2 (R3c → R3c), whereas orthorhombic distortions (R3c → Pbnm) arise in system III at x > 0.2. The magnetic structure is antiferromagnetic (of G type). The samples of systems II and III exhibit weak ferromagnetism at x > 0.2 due to the Dzyaloshinski-Moriya interaction.     

0.2wt% Bi2O3、CuO、B2O3对5Ca0.6La0.267TiO3-5Ca(Mg1/3Nb2/3)O3微波介质陶瓷的性能影响

采用传统的固相反应法,研究了三种烧结助剂Bi2O3、CuO、B2O3对5Ca0.6La0.267TiO3-5Ca(Mg1/3Nb2/3)O3微波介质陶瓷的烧结性能和介电性能的影响。实验结果表明,掺入0.2wt%的Bi2O3、CuO、B2O3产生了液相,有效地降低了体系的烧结温度。Bi2O3和CuO的加入没有改变烧结体的微观形貌,它们介电常数和品质因数随烧结温度的变化趋势和体积密度趋于一致,均在体积密度最大时最高。当温度大于1300℃时,加入0.2wt%B2O3试样有柱状晶体生成,并随着烧结温度的升高而增多,柱状晶体的存在可能促使Q×f值较大的提高,当烧结温度过高时(1350℃),由于柱状晶体过多使得烧结体不均匀导致Q×f值下降。Bi2O3、CuO、B2O3的加入没有改变烧结体的晶相组成,因此所有烧结体均有近零的温度系数。结果表明,加入0.2wt%B2O3的5Ca0.6La0.267TiO3-5Ca(Mg1/3Nb2/3)O3在1325℃烧结温度具有最佳的介电性能:εr=54.87,Q×f=55 726 GHz,τf=-0.6 ppm/℃。     

Refinement of the crystal structure of [Rb x (NH4)1? x ]3H(SO4)2( x = 0.11) by single-crystal X-ray and neutron diffraction: I. Phase II at 300 K

The crystal structure of a mixed [Rb_0.11(NH₄)_0.89]₃H(SO₄)₂ crystal was refined using single-crystal X-ray and neutron diffraction at 300 K. The refinement confirmed that the phase II of the crystal belongs to the monoclinic crystal system (space group C2/c). It was shown that, according to the single-crystal neutron diffraction data, ammonium ions can be represented as regular tetrahedra. The analysis of the constructed difference Fourier maps of the electron and nuclear densities of ammonium ions revealed the presence of additional peaks, which can indicate the dynamic disorder of ammonium ions in the phase II. The results obtained con-firmed that the phase transition to the orientational-glass state can occur in the mixed [Rb_0.11(NH₄)_0.89]₃H(SO₄)₂ crystal with a decrease in the temperature. Original Russian Text ? L.S. Smirnov, K. Wozniak, P. Dominiak, A. Loose, I. Natkaniec, M.V. Frontasyeva, E.V. Pomyakushina, A.I. Baranov, V.V. Dolbinina, 2008, published in Kristallografiya, 2008, Vol. 53, No. 3, pp. 450–459.     

Growth dynamics of Ge1?xSix single crystals obtained by directional constitutional supercooling of the melt

The problem of growth dynamics for crystals of binary solid solutions, obtained by the constitutional supercooling of the melt with a silicon feeding rod, has been solved within the Pfann approximation. The dependences of the change in the axial growth rate of Ge_1−x Si _x crystals (0 ≤ x ≤ 0.3) are calculated. It is shown that the Ge_1−x Si _x crystallization rate significantly changes during growth. The results make it possible to determine the optimal conditions and technological parameters for growing Ge_1−x Si _x single crystals (0 ≤ x ≤ 0.3) with a specified concentration gradient along the crystallization axis.     

<001>-取向（1-x）PbMg1/3Nb2/3O3-xPbTiO3单晶电卡效应的组分依赖性

以<001>取向的PMN-0.14PT,PMN-0.17PT,PMN-0.28PT为代表,研究了PT含量对PMN-x PT单晶电卡效应的影响。通过测试单晶的变温电滞回线,计算了PMN-0.14PT,PMN-0.17PT,PMN-0.28PT的电卡效应。计算结果显示PMN-0.14PT,PMN-0.17PT,PMN-0.28PT单晶的电卡效应最大值随着PT含量的增加而增加,在较小的4 k V/mm电场情况下,分别达到1 K,1.17 K,1.96 K。此外,电卡效应峰发生在居里温度介电峰附近,随着PT含量的增加,电卡效应峰越来越尖锐并移向高温方向。     

X-ray diffraction study of R2[UO2(C3H2O4)2] ·H2O (R = K or Rb)

Compounds K₂[UO₂(C₃H₂O₄)₂] · H₂O (I) and Rb₂[UO₂(C₃H₂O₄)₂] · H₂O (II) are synthesized and their crystal structures are determined by X-ray diffraction. The compounds crystallize in the monoclinic crystal system; for I, a = 7.1700(2) ?, b =12.3061(3) ?, c = 14.3080(4) ?, β = 95.831(2)°, space group P2₁/n, Z = 4, and R = 0.0275; for II, a = 7.1197(2) ?, b = 12.6433(4) ?, c = 14.6729(6) ?, β = 96.353(2)°, space group P2₁/n, Z = 4, and R = 0.0328. It is found that I and II are isostructural. The main structural units of the crystals are the [UO₂(C₃H₂O₄)₂]²⁻ chains, which belong to the AT ¹¹ B ⁰¹ (A = UO₂²⁺, T ¹¹, and B ⁰¹ = C₃H₂O₄²⁻) crystal chemical group of uranyl complexes. The chains and alkali metal ions R (R = K or Rb) are connected by electrostatic interactions and hydrogen bonds. Some specific structural features of [UO₂(C₃H₂O4)₂]²⁻ complex groups are discussed.     

Ionic conductivity of congruently melting Ca0.6Sr0.4F2 and Ca1 ? x ? ySryRxF2 + x (R = La, Ce, Pr, Nd) single crystals with fluorite structure

Single crystals of congruently melting compositions of the Ca_0.6Sr_0.4F₂ and Ca_{1 − x − y} Sr _y R _x F_{2 + x} (R = La, Ce, Pr, Nd; x = 0.16–0.21; y = 0.07–0.16) solid solutions with fluorite structure have been grown by the Bridgman-Stockbarger method. Their electrical properties have been investigated in the range from 473 to 823 K, and it is shown that they are ionic conductors. For Ca_0.6Sr_0.4F₂ crystals, the ionic conductivity σ = 2 × 10⁻⁶ S/cm at 673 K, and the ion transport activation energy E _a = 1.1 eV. For Ca_0.77Sr_0.07La_0.16F_2.16, Ca_0.70Sr_0.11Ce_0.19F_2.19, Ca_0.65Sr_0.15Pr_0.20F_2.20, and Ca_0.58Sr_0.21Nd_0.21F_2.21 crystals, the values of σ lie in the range from 9 × 10⁻⁷ to 2 × 10⁻⁶ S/cm at 500 K, and the activation energy E _a is 0.88–0.93 eV. The concentration and mobility of ionic charge carriers in Ca_{1 − x − y} Sr _y R _x F_{2 + x} crystals have been calculated. Original Russian Text ? N.I. Sorokin, D.N. Karimov, E.A. Krivandina, Z.I. Zhmurova, O.N. Komar’kova, 2008, published in Kristallografiya, 2008, Vol. 53, No. 2, pp. 297–303.     

Infrared transmission of (R2O OR R’O)–(TiO2, Nb2O5 OR Ta2O5)-Al2O3 glasses

The new families of aluminate glasses obtained by the present authors from their melts in the systems K₂O–Ta₂O₅–Al₂O₃, Na₂O–K₂O–Ta₂O₅–Al₂O₃, K₂O –Cs₂O– Ta₂O₅–Al₂O₃, K₂O–Nb₂O₅–Al₂O₃, Na₂Oz.sbnd;K₂O–TiO₂–Al₂O₃, BaO–TiO₂–Al₂O₃, BaO–ZrO₂–TiO₂–Al₂O₃ and Na₂O–K₂O–BaO–ZrO₂–Ta₂O₅–TiO₂ –Al₂O₃ showed high transmissions of visible and infrared (IR) radiation ranging from 0.4 to about 6 μm, as well as high refractive indices up to 2.0. Their physical and chemical properties such as glass-forming ability, softening temperature, hardness and hygroscopicity were comparable to conventional silicate glasses. These properties are useful for IR applications. The cause of the high IR transmission of the aluminate glasses was interpreted in terms of the masses of the constituent cations and the single bond strengths of the cations with oxygen ions.     

Growth and characterization of REBa2Cu3O7−δ superconductor single crystals obtained from xenotime mineral

Single crystals of REBa₂Cu₃O_7−δ (RE=natural mixture of Y and heavy lanthanides in the single crystals) superconductor were successfully grown using rare-earth oxides extracted from xenotime mineral, allowing an alternative and simple route for superconductor single-crystal production. The methodology to extract the rare-earth mixed oxides from the xenotime mineral has three main steps: alkaline fusion, acid lixiviation and oxalic precipitation. Large single crystals with a typical 5×5×0.03 mm³ size were obtained by using a CuO-BaO self-flux and were characterized by scanning electron microscopy (SEM), energy-dispersive electron spectroscopy, X-ray diffraction, magnetic measurements and nanoindentation. The composition of the rare-earth elements of the crystal was different from the starting mixture, possibly due to the different solubilities of the elements in the melt. The final crystal stoichiometry was RE:Ba:Cu=1:2:3. X-ray diffraction analysis showed highly oriented c-axis (c=11.716±0.002 Å). The critical temperature was determined to be around T_C≅88-89 K after the crystals have been submitted to oxygen annealing. Hardness and elastic modulus for ab- and b(a)c-planes were 8.5±0.5 and 160±20 GPa, respectively.     

Mössbauer study in the glass system PbO · 2B2O3 · Fe2O3

The Mössbauer technique has been employed to study the structure and crystallite formation in the glass system PbO · 2B₂O₃ containing upto 30 wt% Fe₂O₃. Like alkali borate glasses, this glass system also exhibits a broadened quadrupole doublet and iron ions are present in Fe³⁺ state. Above about 20 wt%, the crystallites of magnetically ordered states have been identified. Susceptibility variation with concentration suggests the formation of a superparamagnetic state.     

Synthesis and structure of K2[(UO2)4(O)2(OH)2(C2O4)(CH3COO)2(H2O)2]·2H2O

Single crystals of the compound K₂[(UO₂)₄(O)₂(OH)₂(C₂O₄)(CH₃COO)₂(H₂O)₂]·2H₂O (I) are synthesized, and their structure is investigated using X-ray diffraction. Crystals of compound I belong to the triclinic system with the unit cell parameters a = 7.6777(6) ?, b = 7.9149(7) ?, c = 10.8729(9) ?, α = 72.379(2)°, β = 86.430(3)°, γ = 87.635(2)°, V = 628.33(9) ?³, space group P , Z = 1, and R ₁ = 0.0323. The main structural units of the crystals are [(UO₂)₄(O)₂(OH)₂(C₂O₄)(CH₃COO)₂(H₂O)₂]²⁻ chains, which belong to the crystal-chemical group A ₄ M ₂³ M ₂² K ⁰² B ₂⁰¹ M ₂¹ (A = UO₂²⁺, M ³ = O²⁻, M ² = OH⁻, K ⁰² = C₂O₄²⁻, B ⁰¹ = CH₃COO⁻, M ¹ = H₂O) of the uranyl complexes. The chains are formed by linking the centrosymmetric tetramers of the composition (UO₂)₄(O)₂(OH)₂(CH₃COO)₂(H₂O)₂ via tetradentate bridging oxalate ions. The uranium-containing groups are joined into a three-dimensional framework through the electrostatic interaction with potassium cations and a system of hydrogen bonds, which are formed with the participation of atoms involved in the composition of the water molecules, hydroxide ions, and uranyl ions. Original Russian Text ? L.B. Serezhkina, A.V. Vologzhanina, N.A. Neklyudova, V.N. Serezhkin, 2009, published in Kristallografiya, 2009, Vol. 54, No. 3, pp. 483–487.     

Glass formation and structure of glasses in B2O3―Bi2O3―MoO3 system

The purpose of this paper is to study the glass formation tendency in the ternary system B₂O₃―Bi₂O₃―MoO₃ and to define the main structural units building the amorphous network. A wide glass formation area was determined which is situated near the Bi₂O₃―B₂O₃ side. A liquid phase separation region was observed near the MoO₃―B₂O₃ side for compositions containing below 25 mol% Bi₂O₃ and their microheterogeneous structure was observed by SEM. The phase formation was characterized by X-ray diffraction (XRD). By DTA was established the glass transition temperature (T_g) in the range of 380-420 °C and crystallization temperature (T_x) vary between 420 and 540 °C. The main building units forming the amorphous network are BO₃ (1270 and 1200 cm^− 1), BO₄ (930-880, 1050-1040 cm^− 1), MoO₄ (840-760 cm^− 1) and BiO₆ (470 cm^− 1). It was proved that Bi₂O₃ favors the BO₃ → BO₄ transformations while MoO₃ preserves BO₃ units in the amorphous network.     

Crystal and molecular structure of Sr2( Edta ) · 5H2O, Sr2(H2 Edta )(HCO3)2 · 4H2O, and Sr2(H2 Edta )Cl2 · 5H2O strontium ethylenediaminetetraacetates

Three Sr²⁺ compounds with the Edta ⁴⁻ and H₂ Edta ²⁻ ligands—Sr₂(Edta) · 5H₂O (I), Sr₂(H₂ Edta)(HCO₃)₂ · 4H₂O (II), and Sr₂(H₂ Edta)Cl₂ · 5H₂O (III)—are synthesized, and their crystal structures are studied. In I, the Sr(1) atom is coordinated by the hexadentate Edta ⁴⁻ ligand following the 2N + 4O pattern and by two O atoms of the neighboring ligands, which affords the formation of zigzag chains. The Sr(2) atom forms bonds with O atoms of five water molecules and attaches itself to a chain via bonds with three O atoms of the Edta ⁴⁻ ligands. The Sr(1)-O and Sr(2)-O bond lengths fall in the ranges 2.520(2)–2.656(3) and 2.527(3)–2.683(2) ?, respectively. The Sr(1)-N bonds are 2.702(3) and 2.743(3) ? long. In II and III, the H₂ Edta ²⁻ anions have a centrosymmetric structure with the trans configuration of the planar ethylenediamine fragment. The N atoms are blocked by acid protons. In II, the environment of the Sr atom is formed by six O atoms of three H₂ Edta ligands, two O atoms of water molecules, and an O atom of the bicarbonate ion, which is disordered over two positions. In III, the environment of the Sr atom includes six O atoms of four H₂ Edta ²⁻ ligands and three O atoms of water molecules. The coordination number of the Sr atoms is equal to 8 + 1. In II and III, the main bonds fall in the ranges 2.534(3)–2.732(2) and 2.482(2)–2.746(3) ?, whereas the ninth bond is elongated to 2.937(3) and 3.055(3) ?, respectively. In II, all the structural elements are linked into wavy layers. The O-H…O interactions contribute to the stabilization of the layer and link neighboring layers. In III, hydrated Sr²⁺ cations and H₂ Edta ^– anions form a three-dimensional [Sr₂(H₂ Edta)(H₂O)₃] _n ²ⁿ⁺ framework. The Cl⁻ anions are fixed in channels of the framework by hydrogen bonds with four water molecules. In II and III, the N-H groups form four-center N-H…O₃ hydrogen bonds, which include one intermolecular and two intramolecular components. PACS numbers: 61.66.Hq Original Russian Text ? I.N. Polyakova, A.L. Poznyak, V.S. Sergienko, 2009, published in Kristallografiya, 2009, Vol. 54, No. 2, pp. 262–267.     

Structure, thermal, and infrared analyses of [NH3(CH2)3NH3]2[Ni(HP2O7)2(H2O)2] ? 4H2O

[NH₃(CH₂)₃NH₃]₂[Ni(HP₂O₇)₂(H₂O)₂] 4H₂O (NiDAP) is a new diphosphate of transition metallic and organic cations obtained from a mixture of H₄P₂O₇, 2NiCO₃ Ni(OH)₂ 4H₂O and NH₂(CH₂)₃NH₂ in a 1:1/6:1 molar ratio. This mixed organo-mineral compound crystallizes in the triclinic system, P¯, with the unit cell dimensions: a = 7.3678(3)~Å, b = 7.8018(5)Å, c = 11.1958(7)Å, = 76.914(4), = 81.052(4), = 85.46(1), V = 618.57(6)ų and Z = 1. The crystal structure of NiDAP consists of a complex anion, [Ni(HP₂O₇)₂(H₂O)₂]^4– and a diammoniumpropane cation. The complex anion is built up from two neutral water molecules (OW1) and two diphosphosphoric anions coordinated to Ni(II) in a bidentate chelating manner. (OW1) molecules link anionic complexes, [Ni(HP₂O₇)₂(H₂O)₂]^4– to create a thick bidimensional layers parallel to the (a, b) plane. These layers are interconnected in three dimensions through hydrogen bonds established between organic cations, the remaining water molecules OW2, OW3, and some external oxygen atoms of the anionic complex arrays. NiDAP was also characterized by IR spectroscopy, TG-DTA, and DSC analyses.     

EPR study of crystalline and vitreous forms of the Li2OAl2O3SiO2 system

The epr spectra of V⁴⁺ and radiation centres have been studied in β-eucryptite (LiAlSiO₄), β-, γ-spodumene (LiAlSi₂O₆) and in glasses prepared by the fusion of the single crystals. It is shown that the electronic structures of the vitreous state in the Li₂OAl₂O₃SiO₂ system and that of the crystalline forms differ considerably. The change of the electronic structure on crystallization is not direct, but is realized through the intermediate state whose electronic structure differs from that of glasses and crystals.     

Synthesis and Crystal Structure of a New Mixed Alkali Oxalate A1?x(NH4)x(H2C2O4)(HC2O4)(H2O)2 with A?=?K, Rb

Abstract  

In the present study we report the synthesis, crystal structure, spectroscopic and thermal analysis of mixed alkali A_1−x (NH₄) _x (H₂C₂O₄)(HC₂O₄)(H₂O)₂ with A = K, Rb. Single crystal refinements showed that the two compounds Rb_0.86(NH₄)_0.14(H₂C₂O₄)(HC₂O₄)(H₂O)₂ and K_0.53(NH₄)_0.47(H₂C₂O₄)(HC₂O₄)(H₂O)₂ adopt P − 1 space group. The nine fold coordination cationic sites are randomly occupied by Rb⁺/NH₄ ⁺ and K⁺/NH₄ ⁺ respectively for rubidium and potassium compounds. The structure consists of a three-dimensional network formed by the succession along (c) axis of corrugated sheets formed by alkali polyhedra layers in the packing of four-membered rings [A₄(HC₂O₄)₂(H₂C₂O₄)₂] linked and bridged by oxalate groups that behave as bi and tetradentate ligand under three different coordination modes. The stability of the crystal lattice is ensured by interesting hydrogen bonding contacts: N–H···O and O–H···O. The thermal behavior under air reveals two anomalies at 368 and 402 K attributed respectively to a structural phase transition probably due to the reorientation of ammonium tetrahedral and to the release of crystalline water. IR spectroscopy further confirms that this material loses crystallization water gradually in the temperature range 400–460 K.