Browsing by Autor "P. Grima"

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ALEACIONES DE (CuAlSe2)1-X (TaSe)X (0 ≤ X ≤ 0.5): MEDIDAS DE DIFFRACCION DE RAYOS X, CALORIMETRIA DIFERENCIAL DE BARRIDO Y MICROSCOPIA ELECTRONICA DE BARRRIDO
(2021) P. Grima; Marcos Muñoz; Sonia Durán; Gerzón E. Delgado; Eduardo Pérez Cappe; Jennifer A. Aitken; Dibya Prakash Ray
Polycrystalline samples belonging to the (CuAlSe2)1-x (TaSe)x alloys system, in the composition interval 0 ≤ x ≤ 0.5, were synthesized by the melt and anneal method. X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Differential Thermal Analysis (DTA) techniques were used for the characterization of the products. From XRD it can be observed a chalcopyrite-like single phase in the composition range 0≤ x ≤0.1, with lattice parameters very close to CuAlSe2 and two-phases, chalcopyrite-like and hexagonal-Cu0.52TaSe2, for 0.1 < x ≤ 0.5. By SEM, the stoichiometry for all samples had been measured and the new phase was identified as (Cu0.4Al0.3)TaSe2 . From DTA the respective phase transitions with temperature were obtained. Using the experimental information, a preliminary T-x phase diagram was proposed.
COEXISTENCE OF SUPERPARAMAGNETIC AND FERROMAGNETIC COMPONENTS IN (CuGa) 1-x Fe x Se 2-x SOLID SOLUTIONS WITH x=0.1, 1/3 and 1/2 ( COEXISTENCIA DE COMPONENTES FERROMAGNÉTICA Y SUPERPARAMAGNÉTICA EN SOLUCIONES SÓLIDAS (CUGA) 1-X FE X SE 2-X CON X=0.1, 1/3 Y 1/2 )
(2018) P. Grima; Luis Nieves; Miguel Angel Alfaro Soto; Miguel Quintero; Humberto Cabrera; Inti Zumeta‐Dubé; Alejandro Rodríguez; Jennifer A. Aitken; P. Raics
The magnetic behavior of (CuGa) 1-x Fe x Se 2-x solid solutions with compositions x=0.1, 1/3 and 1/2 has been investigated using SQUID techniques. DC magnetic susceptibility was measured in the temperature range 2 < T < 300K using Zero Field Cooling (ZFC) – Field Cooling (FC) protocol. It has been observed that the experimental curves can be analyzed as the sum of two magnetic components, one superparamagnetic and the other one ferromagnetic. The ferromagnetic component persist up to temperatures higher than room temperature making this report the first observation of room temperature ferromagnetism in A I B III C VI 2 compounds doped (or alloyed)with transition metals. El comportamiento magnetico de las soluciones solidas (CuGa) 1-x Fe x Se 2-x con composiciones x=0.1, 1/3 y 1/2 fue investigado por medio de la tecnica de SQUID. Las susceptibilidades DC fueron medidas en el rango de temperatura 2 < T < 300K usando el protocolo Enfriamiento a Campo Zero (ECZ) – Enfriamiento con Campo (EC). Se observo que las curvas experimentales podian ser analizadas como la suma de dos componentes magneticas, una superparamagnetica y otra ferromagnetica. La componente ferromagnetica persiste a temperaturas por arriba de la temperatura ambiente siendo este reporte la primera observacion de ferromagnetismo a temperatura ambiente en compuestos A I B III C VI 2 dopados (o aleados) con metales de transicion.
Crystal structure of the quaternary compounds CuFe2AlSe4 and CuFe2GaSe4 from X-ray powder diffraction
(Springer Science+Business Media, 2015) Gerzón E. Delgado; Asiloé J. Mora; P. Grima; M.A. Morris; S. Durán; M. Quintero; J.M. Briceño
CRYSTAL STRUCTURE OF THE QUATERNARY SEMICONDUCTOR COMPOUND CuFeCrSe3
(2019) Gerzón E. Delgado; Pilar Delgado–Niño; P. Grima
The compounds with ternary structures of the chalcopyrite family Cu-III-Se2 (III = Al, Ga, In, Cr) form a wide group of semiconductor materials with diverse optical and electrical properties, and the addition of FeSe binary compound produces alloys of the type (Cu-III-Se2)1-x(Fe-Se)x. These types of materials have received increasing attention as promising thermoelectric materials due to their high efficiency, tunable transport properties, high elemental abundance and low toxicity. This work aims to synthesize and characterize structurally a new material belonging to this semiconductor system with x = ½, the quaternary compound CuFeCrSe3. This material was prepared by the melt and anneal technique and its structure was refined from Xray powder diffraction pattern using the Rietveld method. The X-ray powder pattern was mainly composed of CuFeCrSe3 (79.1%) with CuCr2Se4 (20.9%) appearing as secondary phase. The principal phase crystallizes with tetragonal symmetry in the space group P 4 2c (Nº 112), Z = 1, with a = 5.5082(2) Å, c = 10.943(1) Å, V = 332.01(1) Å3. The refinement of 18 instrumental and structural parameters led to Rp = 8.8 %, Rwp = 9.1 %, Rexp = 7.8 % and S = 1.2. This material, belonging to the semiconductor system I-II-III-VI3, crystallizes with a CuFeInSe3-type structure in a normal adamantane-structure. CuFeCrSe3 is a new semiconductor compound, related to the superionic phase CuCrSe2, and can be considered as a potential candidate for mediumtemperature thermoelectric applications.
DIAGRAMA DE FASES DE LAS ALEACIONES (CuGaSe 2 ) 1-x (FeSe) x (0≤x≤0.5) ( PHASE DIAGRAM OF (CuGaSe 2 ) 1-x (FeSe) x (0≤x≤0.5) ALLOYS )
(2012) Miguel Humberto Soto Dávila; P. Grima; Miguel Quintero; Menjamín Salas; Marcos Muñoz; Sonia Durán; Luis Nieves; Ekadink Moreno; Miguel Ángel Sánchez Ramos; J.M. Briceño
Se prepararon muestras policristalinas del sistema de aleaciones (CuGaSe 2 ) 1-x (FeSe) x en el rango de composiciones (0≤x≤0,5) por la tecnica de fusion y recocido. Los productos fueron analizados usando las tecnicas de Difraccion de Rayos X (DRX) y Analisis Termico Diferencial (ATD). Del analisis de los resultados experimentales se propone un diagrama de fases, T-x, en la region de composiciones cercana al CuGaSe 2 . Polycrystalline samples of the (CuGaSe 2 ) 1-x (FeSe) x alloy system in the composition range (0≤x≤0.5) were prepared by the melt and anneal technique. X-Ray Diffraction (XRD) and Differential Thermal Analysis (DTA) techniques were used to characterize the products. From the analysis of the experimental results a T-x phase diagram is proposed in the CuGaSe 2 -rich field (0≤x≤0.5).
Diseño, Fabricación y Caracterización de Celdas Solares p-CdTe/n-CdS con Películas Delgadas
(University of Zulia, 2021) P. Grima; B. Conquet; Elías Rafael Sánchez Verde; Miguel A. Contreras-Rangel; Luis Nieves; Ana del Valle Velasquez-Velasquez; William Jose González-Escalona; Luis Alfonso Ruiz Echenagucia; Luis Nieves; Universidad de los Andes
Venezuela, debido a su ubicación geográfica, riqueza en fuentes y recursos naturales, tiene un valioso potencial para el uso de energías renovables, especialmente la solar. Gran parte de su territorio se caracteriza por una insolación media por encima de 3 Kwh/m2, considerada apta para la generación fotovoltaica. El CdTe es un conocido semiconductor II-VI, que se ha consolidado como líder en la industria fotovoltaica, posee propiedades óptimas para las celdas solares. Esta investigación se enfocó en diseñar, fabricar y caracterizar celdas solares p-CdTe/n-CdS. El diseño se realizó buscando el modelo más simple y de menor costo. La fabricación se llevó a cabo en un equipo Ortus-700, usando la técnica de evaporación térmica. Para la caracterización de las celdas, se aplicaron las técnicas de difracción de rayos-X, absorción óptica y análisis de característica corriente-voltaje. Evidenciándose una orientación preferencial típica de una estructura cúbica de la Zinc Blenda, valores de brecha de energía directa entre 2,15 a 2,85 eV y un rendimiento similar entre las celdas solares preparadas. Se logró la fabricación y caracterización de nueve celdas p-CdTe/n-CdS, mediante la formación de una unión p-n; con eficiencias de ~ 3 %.
Optical Absorption and Raman Scattering Measurements in CuAlSe<sub>2</sub> at High Pressure
(Wiley, 1996) L. Roa; J. C. Chervin; A. Chévy; M. E. Dávila; P. Grima; J. Gonzáez
Abstract The ternary compound CuAlSe 2 is a direct energy gap semiconductor ( E g = 2.6 eV at 300 K) crystallizing in the tetragonal chalcopyrite structure. In this work the optical absorption edge and the Raman active modes of CuAlSe 2 were measured as a function of pressure up to 30 GPa. The measurements were performed in a membrane diamond‐anvil cell at ambient temperature using neon gas as pressure transmitting medium. The direct energy gap (Γ → Γ ) increases linearly with pressure at the rate of 4.7 × 10 −2 eV GPa −1 . At 6.7 GPa the character of the fundamental gap changes to pseudodirect (Γ → Γ ). This gap decreases with pressure at a rate of −2.9 × 10 −2 eV GPa −1 . The effect of pressure on the phonon frequencies is discussed in terms of the Grüneisen parameters. A first‐order structural phase transition was observed at 12 GPa in the upstroke and at 2 GPa in the downstroke.
PHASE DIAGRAM OF (CuInTe 2 ) 1-x (FeTe) x ALLOYS (0 ≤ x ≤ 0.6) ( DIAGRAMA DE FASES DEL SISTEMA DE ALEACIONES (CuInTe 2 ) 1-x (FeTe) x (0 ≤ x ≤ 0.6)
(2017) P. Grima; M. Quintero; Luis Nieves; Humberto Cabrera; Eduardo Pérez Cappe; Inti Zumeta‐Dubé; Jennifer A. Aitken; Jacilynn A. Brant
Polycrystalline samples of the (CuInTe2)1-x (FeTe)x alloy system where prepared by the melt and anneal technique and the products characterized by X-Ray Diffraction (XRD), Differential Thermal Analysis (DTA) and Optical Diffuse Reflectance Spectroscopy (ODRS) techniques. A first sight, samples were composed by a tetragonal phase with traces of a secondary phase identified as Fe1.125Te. Under exhaustive examination, it was found that the tetragonal phase, at the same time, is composed by two phases, with the same crystal structure and close lattice parameters, typical of spinodal decomposition. From ODRS measurements, optical absorption vs energy curves was obtained. These curves show how the addition of Fe produces a disordering-reordering process in the cationic sublattice suggesting that composition x=0.5 (CuFeInTe3) could be an intermediate compound in the diagram. Using the obtained information, a preliminary T-x phase diagram is proposed. Se prepararon muestras policristalinas del sistema de aleaciones (CuInTe2)1-x (FeTe)x por la tecnica de fusion y recocido; los productos fueron caracterizados por las tecnicas de Difraccion de Rayos X (DRX), Analisis Termico Diferencial (ATD) y Reflectancia por Espectroscopia de Optica Difusa (REOD). A primera vista, las muestras se componen de una fase tetragonal con trazas de una fase secundaria identificada como Fe1.125Te. Bajo un examen mas exhaustivo, se encontro que la fase tetragonal se compone a su vez de dos fases, con la misma estructura cristalina y parametros de red cercanos, tipico de una descomposicion espinodal. A partir de mediciones REOD, se obtuvieron las curvas de absorcion optica vs energia. Estas curvas muestran como la adicion de Fe produce un proceso de desorden-reorden en la subred cationica que sugiere que la composicion x = 0,5 (CuFeInTe3) podria ser un compuesto intermediario en el diagrama. Con la informacion obtenida, se propone un diagrama de fases T-x.
PREPARATION, CRYSTAL STRUCTURE, THERMAL ANALYSIS, SCANNING ELECTRON MICROSCOPY AND OPTICAL BAND-GAPS OF Cu 2 GeTe 4 AND Cu 2 SnTe 4 ALLOYS ( PREPARACION, ESTRUCTURA CRISTALINA, ANALISIS TERMICO, MICROSCOPIA ELECTRONICA DE BARRIDO Y BRECHA OPTICA DE ENERGIA DE LAS ALEACIONES Cu 2 GeTe 4 Y Cu 2 SnTe 4 )
(2015) P. Grima; Ricardo Peña; Luis Nieves; G. Marcano; Miguel Quintero; Ekadink Moreno; Jacilynn A. Brant; Jennifer A. Aitken
Polycrystalline samples (weight ~ 1g) of Cu 2 GeTe 4 and Cu 2 SnTe 4 alloys were prepared by the usual melt and anneal method and the products characterized by X-Ray Diffraction (XRD), Differential Thermal Analysis (DTA), Scanning Electron Microscopy (SEM) and Optical Diffuse Reflectance UV/VIS/NIR Spectroscopy techniques. It was found that: a) Cu 2 GeTe 4 and Cu 2 SnTe 4 crystallize in an orthorhombic structure (s.g. Imm2; N o 44) with lattice parameters a=5.9281(4) A, b=4.2211(6) A, c=12.645(5) A and a=6.0375(6) A, b= 4.2706 (3) A, c=12.844(1) A, respectively; b) both alloys show two thermal transitions: 762 and 636K upon heating and; 700 and 578K upon cooling for Cu 2 GeTe 4 ; 702 and 636K upon heating and; 650 and 590K upon cooling for Cu 2 SnTe 4 ; c) both alloys present large deviations of stoichiometry for the cations Cu (~35%), Ge (7.2%) and Sn (26.4%) and minor deviation within the experimental error, for the anion Te; and, d) the measured optical band gaps were 0.63 and 0.53 eV for Cu 2 SnTe 4 and Cu 2 GeTe 4 , respectively. Se prepararon muestras policristalinas (peso ~ 1 g) de las aleaciones Cu 2 GeTe 4 y Cu 2 SnTe 4 por el metodo de fusion y recocido y los productos caracterizados por las tecnicas de Difraccion de Rayos X (DRX ) , Analisis Termico Diferencial (ATD), Microscopia Electronica de Barrido (MEB) y espectroscopia de reflectancia optica difusa UV / VIS / CIR. Se encontro que : a) Cu 2 GeTe 4 y Cu 2 SnTe 4 cristalizan en una estructura ortorrombica ( g.e. Imm2; N o 44 ) con parametros de red a = 5,9281 (4) A , b = 4.2211 (6) A , c = 12.645 (5) A y a = 6.0375 (6) A , b = 4,2706 (3) A , c = 12.844 (1) A , respectivamente ; b) ambas aleaciones muestran dos transiciones termicas : 762 y 636K al calentar y ; 700 y 578K tras el enfriamiento para Cu 2 GeTe 4 ; 702 y 636K al calentar y ; 650 y 590K tras el enfriamiento para Cu 2 SnTe 4 ; c) ambas aleaciones presentan importantes desviaciones estequiometricas en sus cationes: Cu (~35%), Ge (7.2%) and Sn (26.4%) y menor que el error experimental para el anion Te; y d) las brechas opticas de energia medidas fueron 0.63 y 0.53 eV para Cu 2 SnTe 4 y Cu 2 GeTe 4 , respectivamente.
PROPIEDADES MAGNÉTICAS DEL SISTEMA DE ALEACIONES Cu 2 Cd 1−Z Mn Z SnSe 4 ( MAGNETIC PROPERTIES OF THE ALLOY SYSTEM Cu 2 Cd 1−z Mn z SnSe 4 )
(2018) Ekadink Moreno; Miguel Quintero; M. Morocoima; A U Manuel Villarreal; Eugenio Quintero; P. Grima; R. Tovar; D. Caldera; Silvana Alvarez; P. Bocaranda
En este trabajo se reportan las medidas de susceptibilidad magnetica c en funcion de la temperatura en el rango de 2-300 K de muestras policristalinas del sistema de aleaciones semiconductor magnetico Cu 2 Cd 1 − z Mn z SnSe 4 . La constante de Curie C aumenta linealmente con la concentracion z. La temperatura de Curie-Weiss q y la temperatura de Neel T N aumentan linealmente con la concentracion z, debido a un cambio de orden parcial a desorden total de los atomos de Mn 2+ en el sistema. En una primera aproximacion, se puede considerar que el sistema de aleaciones tiene un comportamiento tipico de vidrio de espin, lo que implica que los iones de Mn 2+ se encuentran de alguna manera desordenados en la red cationica del material. Measurements of magnetic susceptibility c as a function of temperature in the range 2-300 K on polycrystalline samples of Cu 2 Cd 1 − z Mn z SnSe 4 magnetic semiconductor alloy system are reported. The Curie constant C increase linearly with z. The Curie-Weiss temperature θ and Neel temperature T N increase linearly with z due to a partial order change to total disorder of the Mn +2 ions in the system. In a first approximation, it can be considered that this system has typical spin glass behavior (spin glass system), which means that ions Mn +2 are somewhat disorderly way cationic network of material.
SÍNTESIS, CARACTERIZACIÓN ESTRUCTURAL Y MAGNÉTICA DE LA ALEACIÓN CuAl 0.67 Cr 0.33 S 2 ( SYNTHESIS, STRUCTURAL AND MAGNETIC CHARACTERIZATION OF THE ALLOY CuAl 0.67 Cr 0.33 S 2 )
(2011) Manuel Antonio Villarreal Uzcátegui; P. Grima; Miguel Quintero; Ekadink Moreno; Gerzón E. Delgado; Jorge Fernández; Pedro Silva; José Luis Villca Villegas
En este trabajo se reporta la sintesis, la caracterizacion estructural y la caracterizacion magnetica de la aleacion CuAl 0.67 Cr 0.33 S 2 . La muestra fue sintetizada utilizando la tecnica de fusion directa de los elementos constituyentes. El analisis quimico (EDX) permitio establecer las relaciones estequiometricas para esta aleacion. El patron de difraccion de polvo fue indexado y la fase principal cristaliza con simetria tetragonal y parametros de celda unidad a = 5.3410(1) A, c = 10.4154(2) A. Este material es isomorfo con la estructura calcopirita que cristaliza en el grupo espacial. Las medidas de susceptibilidad magnetica y resonancia paramagnetica electronica, en funcion de la temperatura, fueron realizadas en un magnetometro SQUID y en un espectrometro BRUKER EMX de banda X, respectivamente. La aleacion exhibe un ordenamiento Ferrimagnetico con una temperatura de Neel ~ 40 K. El ancho de linea EPR muestra un comportamiento paramagnetico entre 100 y 600 K, mientras que el campo de resonancia y el factor g muestran una ligera variacion con la temperatura. This paper reports the synthesis, structural and magnetic characterization of the alloy CuAl 0.67 Cr 0.33 S 2 . The sample was synthesized using the technique of direct fusion of the constituent elements. Chemical analysis (EDX) allowed establishing the stoichiometric ratios for this alloy. The powder diffraction pattern was indexed and the main phase crystallizes with tetragonal symmetry and unit cell parameters a = 5.3410(1) A, c = 10.4154(2) A. This material is isomorphic with the chalcopyrite structure which crystallizes in space group. Measurements of magnetic susceptibility and electronic paramagnetic resonance, depending on temperature, were performed in a SQUID magnetometer and a Bruker EMX X-band, respectively. The alloy exhibits a ferrimagnetic ordering with Neel temperature ~ 40 K. The EPR linewidth shows a paramagnetic behavior between 100 and 600 K, while the resonance field and the g factor show a slight variation with temperature.
STRUCTURAL CHARACTERIZATION OF THE NEW DIAMOND-LIKE SEMICONDUCTOR CuNbGaSe3
(2018) Jesús Alberto Flores Cruz; Gerzón E. Delgado; Jines E. Contreras; M. Quintero; Luis Nieves; P. Grima
The chalcogenide compound CuNbGaSe3, belonging to the system I-II-III-VI3, has been investigated by means of X-ray powder diffraction and its crystal structure has been refined by the Rietveld method.This is a material of the semiconductor type, which improves the properties of a simple semiconductor like CuGaSe2 because it ads spintronic applications due to its magnetic behavior. The powder pattern was composed by 94.2% of the principal phase CuNbGaSe3 and 5.8% of the secondary phase Cu0.667NbSe2. This material crystallizes with a CuFeInSe3-type structure in the tetragonal space group P4 2c (Nº 112), unit cell parameters a = 5.6199(4) Å, c = 11.0275(2) Å, V = 348.28(4) Å3, with a normal adamantane-structure where occurs a degradation of symmetry from the chalcopyrite structure I4 2d to a related structure P4 2c.
Synthesis and crystal structure analysis of the magnetic ternary compound Mn2GeTe4
(Universidad de Los Andes, 2015) Gerzón E. Delgado; Luis Betancourt; Asiloé J. Mora; Jines E. Contreras; P. Grima; Miguel Quintero
Samples of the ternary phase Mn2GeTe4 were synthesized by direct fusion using the anneal method. X-ray powder diffraction results indicated that this material crystallizes with orthorhombic symmetry, space group Pnma, and unit cell parameters: a = 13.950(2) Å, b = 8.115(1) Å, c = 6.592(1) Å, V = 746.2(2) Å3. The Rietveld refinement converged to the figures of merit Rp = 6.9%, Rwp = 8.6%, Rexp = 5.6% and X2 = 2.4. Mn2GeTe4 is the first telluride of the II2-IV-VI4 family that crystallizes with olivine-type structure.
SYNTHESIS, CRYSTAL STRUCTURE AND MAGNETIC BEHAVIOR OF CuCo 2 InTe 4 AND CuNi 2 InTe 4 ( SÍNTESIS, ESTRUCTURA CRISTALINA Y COMPORTAMIENTO MAGNÉTICO DE CuCo 2 InTe 4 Y CuNi 2 InTe 4 )
(2016) P. Grima; Miguel Angel Alfaro Soto; Orlando Izarra; Luis Nieves; Miguel Quintero; Gerzón E. Delgado; Humberto Cabrera; Inti Zumeta‐Dubé; Alejandro Rodríguez; Jennifer R. Glenn
Polycrystalline samples of nominal CuCo 2 InTe 4 and CuNi 2 InTe 4 were prepared by the melt and anneal method and the products characterized by powder X-ray diffraction and SQUID techniques. It was found that CuCo 2 InTe 4 and CuNi 2 InTe 4 crystallize in the tetragonal space group (No 121), Z = 2, in a stannite-type structure, with the binaries CoTe and NiTe as secondary phases, respectively. The magnetic behavior of CuCo 2 InTe 4 is that of a superparamagnetic state with an irreversibility temperature of ~450K and a maximum coercive field of 35 Oe at 2K; whereas CuNi 2 InTe 4 shows two magnetic components, one diamagnetic an another weak ferromagnetic. Se prepararon muestras policristalinas de CuCo 2 InTe 4 and CuNi 2 InTe 4 en su valor nominal por el metodo de fusion y recocido, y los productos fueron caracterizados por las tecnicas de difraccion de rayos X y SQUID. Se encontro que CuCo 2 InTe 4 and CuNi 2 InTe 4 cristalizan en una estructura tetragonal, grupo espacial (No 121), Z = 2, tipo estannita, con presencia de los binarios CoTe y NiTe como fases secundarias, respectivamente. El comportamiento magnetico de CuCo 2 InTe 4 es de tipo superparamagnetico con una temperatura de irreversibilidad de ~450K y un campo coercitivo maximo de 35 Oe a 2K; mientras que CuNi 2 InTe 4 presenta dos componentes magneticas, una diamagnetica y otra ferromagnetica debil.
Synthesis, X-ray diffraction and Magnetic measurements of Cu (Ni, Co) In S4 alloys: Superconductor behavior of CuCo2InS4
(2020) P. Grima; Gerzon Delgado; Eduardo Pérez Cappe; Jennifer A. Aitken; Dibya Prakash
Polycrystalline samples of CuCo2InS4 and CuNi2InS4 alloys were prepared by the melt and anneal method. The products were characterized by X-Ray Diffraction (XRD) and Superconducting Quantum Interference Device (SQUID) techniques. From XRD it was found that both alloys, CuCo2InS4 and CuNi2InS4, are composed by two orthorhombic phases. In the case of CuCo2InS4, the crystal structure of the main phase has been refining (Rietveld method) as Pmn21 (N 31) with lattice parameters a=8.100(1) A, b=7.116(8) A and c=6.391(6) A; for CuNi2InS4, the lattice parameters of the two phases were indexed (DICVOL 06). With respect to the magnetic behavior, CuCo2InS4 is a superconductor with Tc=3.2 K whereas CuNi2InS4 is a high temperature (Tc ~ 380 K) weak ferromagnetic with abnormal temperature dependence of the coercitive field.