Crystal Structure and Magnetic Properties of the New Zn1.5Co1.5B7O13Br Boracite
Roberto Escudero, Francisco Morales, Marco A Leyva Ramirez, Jorge Campa-Molina, S. Ulloa-Godinez
aa r X i v : . [ c ond - m a t . m t r l - s c i ] J un Crystal Structure and Magnetic Properties of the NewZn . Co . B O Br Boracite
Roberto Escudero ∗ and Francisco Morales Instituto de Investigaciones en Materiales,Universidad Nacional Aut´onoma de M´exico. A.Postal 70-360. M´exico, D.F., 04510 M ´EXICO.
Marco A Leyva Ramirez
Departamento de Qu´ımica, Centro de Investigaci´on yEstudios Avanzados del IPN. 07360, M´exico, D. F.
Jorge Campa-Molina and S. Ulloa-Godinez
C. Universitario de Ciencias Exactas e Ingenieras,Universidad de Guadalajara, Laboratorio de Materiales AvanzadosDepartamento de Electronica. 044840, Guadalajara, Jalisco. M ´EXICO. (Dated: September 26, 2018) bstract New Zn . Co . B O Br boracite crystals were grown by chemical transport reactions in quartzampoules, at a temperature of 1173 K. The crystal structure was characterized by X-ray diffraction.The crystals present an orthorhombic structure with space group Pca2 , (No. 29). The determinedcell parameters were: a = 8.5705(3)˚A, b = 8.5629(3) ˚A, and c = 12.1198(4)˚A, and cell volume,V = 889.45(5) ˚A with Z = 4. Magnetic properties in single crystals of the new boracite, weredetermined. The Susceptibility-Temperature ( χ − T ) behavior at different magnetic intensities wasstudied. The inverse of the magnetic susceptibility χ − ( T ) shows a Curie-Weiss characteristic withspin s = 3 / l . At low temperatures, below 10 K, χ ( T ) showsirreversibility that is strongly dependent on the applied magnetic field. This boracite is ferrimag-netic up to a maximum temperature of about 16 K, as shows the coercive field. The reduction ofthe irreversibility by the influence of the magnetic field, may be related to a metamagnetic phasetransition. PACS numbers: Boracites; Ferromagnetism; Metamagnetic transitions; Schottky anomaly; Crystal structure ∗ [email protected] . INTRODUCTION The term boracites is at present given to more than 25 isomorphous compounds all withthe general formula Me B O X, where Me is one of the divalent metals Mg, Cr, Mn, Fe,Co, Ni, Cu, Zn or Cd and X is usually Cl, Br or I. Occasionally X can be OH, S, Se or Teand monovalent lithium substitutes for Me; then the formula becomes Li B O X where Xis a halide [1, 2]. Natural and synthetic boracites have attracted the attention of researcherssince the early times R. J. Ha¨uy [3, 4]. The mineral boracite Mg B O Cl, provides the nameof this large family [2, 3]. Only other four natural boracites are known: ericaite, chambersiteMn B O Cl [5], congolite Fe B O Cl [6], and trembathite (Mg,Fe) B O13Cl [6]. Ericaiteand trembathite are natural mixed boracites, which have been taken as motivation on thisinvestigation to synthesize other types of mixed boracites. The reason to synthesizing themixed (Zn,Co) B O Br boracites, is related to the idea to understand more about thephysical and chemical properties, when combining metals. Boracites have received specialattention because of their unusual physical properties [7–10]. In this contribution we presentresults on the crystallographic characteristics and mainly on the new magnetic propertiespresented in Zn . Co . B O Br. We report the magnetic characteristics from room to lowtemperatures, investigating the influence of the magnetic field, and the behavior of thespecific heat at low temperatures. We found that irreversibility in χ ( T ) is strongly dependenton the intensity of the applied magnetic field. We explain this behavior as the transitionfrom low to high spin changes (metamagnetic transition). So, at low field the materialbehaves as a ferrimagnet, and transits to a ferromagnetic state (high spin) with increasingmagnetic intensity. In addition specific heat measurements show a Schottky anomaly at lowtemperature. We interpreted this anomaly as a resonance between the splitting of spins,from ms states: 3/2 to -3/2, and the thermal energy. II. EXPERIMENTAL DETAILSA. Crystal growth
The Schmid method for the sample preparation of crystalline materials was followed [11].The compound was grown by chemical reaction of vapour phases. Reactants were placedin three fused quartz crucibles of different dimensions separated by small quartz rods, and3ertically aligned. To facilitate the chemical transport, the initial compounds (halogenures,metal oxides, and boric acid) were placed in the bottom, first crucible. The content of thiswas 1.7 g of B O (obtained by dehydrating H BO ); the second crucible contains 0.5 gof ZnO and CoO. Finally, the upper third crucible contains 0.8 g of each divalent metalhalides; CoBr , and ZnBr . These were placed inside a quartz ampoule under vacuum. Thechemical transport reactions were carried out by heating the ampoule in a vertical oven withthe following heating steps: 1173 K over a period of 50 hours, and 913 K over a period of20 hours. At the end of this process, once at room temperature we observed small singlecrystals boracites. The single crystals of the mixed boracites show an intense purple colorand approximately 3 mm in size, those crystals mostly are formed at the end of the lowercrucible. B. Crystal structure analysis
X-ray data were collected using graphite monochromated MoK α radiation λ = 0 . C. Refinement details
Refinement of Boracites by single-crystal techniques is complicated because the veryfrequent twins created in the growing process [14]. In order to solve this complication weused a twin matrix method to refine the crystal structure. We detected two domains inthe studied single crystal by using the package PLATON [15]. In order to have a betterinterpretation of the studied electronic densities a Multi-Scan absorption correction was usedwith the SADABS software [13]. Due to high electron density of the Bromine atoms, thelocalization of light elements becomes complicate, and also the corresponding refinement:This fact explains the low isotropic temperature factor(Uiso) in the Boron and Oxygenatoms. Accordingly, the X-ray pattern of the studied sample was refined isotropically. Inthis case was necessary to consider a restrain for the occupation. This was taken as 0.5 ofZn/Co atoms; with this restrain we obtained good agreement for the anisotropical values4or Zn and Co atoms. thus, good convergence and low R value were obtained.
D. Magnetic measurements
Magnetic measurements were performed using a MPMS magnetometer (Quantum De-sign). M − T measurements were obtained at three different magnetic intensities; 100, 1000,and 5000 Oe. The data acquisition modes were the standard Zero Field Cooling (ZFC)and Field Cooling (FC). After these measurements we obtained the magnetic susceptibilityas a function of the temperature χ ( T ) in terms of cm /mol, and the Pascal constant wereadded. In addition, we performed isothermal magnetization measurements cycles at varioustemperatures. E. Specific heat measurements
Specific heat measurements C P as a function of temperature were performed using athermal relaxation method, utilizing a PPMS (Quantum Design) apparatus. Measurementswere performed at low temperatures with applied magnetic field of 0, 100 and 5000 Oe. The C P values were corrected subtracting the addenda due to the sample support and the greaseused to glue the sample on the support. III. RESULTS AND DISCUSSIONA. Structural characterization
X-ray diffraction analysis reveals that Zn . Co . B O Br compound crystallizes in anorthorhombic structure with space group Pca2 (No. 29). The structural parameters de-termined are summarized in I. In Supplementary Material the final refined positional andthermal parameters are given in Table 2, and the main interatomic distances and angles aregiven in Table 3. The unit cell for this boracite is shown in 1.The bond distances and angle values on first approach, looks abnormal. The reasonis because the difference between the coordination polyhedral bond distance is too big.For instance, the octahedral polyhedral whose central atoms are Br, is coordinated withsix Zn or Co atoms; the average bond distances Zn(1)-Br and Co(1)-Br are 2.576(4) and5 ABLE I. Crystal data and data parameters of Zn . Co . B O Br boracite.Compound BoraciteChemical formula Co . Zn . B O BrFormula weight 550.03Cryst size [mm] 0.075 x 0.11 x 0.19Cryst. system OrthorhombicSpace group Pca2 a , [˚A] 8.5705(3) b , [˚A] 8.5629(3) c , [˚A] 12.1198(4)V, [˚A ] 889.45(5)Z 4 ρ (calc.), [Mg/m ] 4.107 µ [mm − ] 11.366F(000) 1038Index range − ≤ h ≤ − ≤ k ≤ − ≤ l ≤ θ [ ◦ ] 54.80Temp, [K] 293(2)Refl. collected 10425Refl. unique 2027Refl. observed (4 σ ) 1273R (int.) 0.0601No. variables 116W. scheme x/y w − = σ F + ( xP ) + yPP = ( F + 2 F C ) / σ ) 0.0334Final wR2 0.0708Larg. res. peak [e/˚A ] 3.140 IG. 1. (Color online) This figure displays the Unit cell for Zn . Co . B O Br boracite. Atomsare marked in different colors and descending in size, as: blue balls, Zn; green balls, Co; orange,Br; yellow, B; and small red ones, Oxygens. a and b is 12.1151 ˚A, which is almost the same value of the c parameter(12.1198 ˚A). Likewise, observing the bond distance between different tetrahedra, as BO ,we clearly can distinguish very strong deformations. These deformations may provokes thatthe total atom charge in the crystal structure not be neutralized, and consequently zoneswith negative and positive charge could appear. This distortion generates electric dipolesalong the preferred direction of the crystal structure. B. Magnetic behavior χ ( T ) presents a paramagnetic behaviorat high temperature, the values are quite small and close to zero. At low temperature,about 50 K the χ value gradually increases and at about 12 K χ presents a rapid increase;at 2 K has a huge value of about 4 cm /mol . However, in the ZFC mode at 2 K also, χ isnegative but rapidly grows as the temperature rises, the a maximum values at 11 K is about0.23 cm /mol . This is small in comparison to the maximum value in FC mode. A thatmaximum the two modes present an irreversible behavior when the temperature decreases.This behavior clearly is displayed in the insert of 2. 3 shows the inverse susceptibility χ − ( T ),7 ( c m / m o l ) T (K) FC ( c m / m o l ) T (K)ZFCFC
FIG. 2. (Color online) Magnetic susceptibility vs. Temperature, χ ( T ) of (Zn,Co) B O Br bo-racite single crystal measured with 100 Oe, in ZFC and FC modes, the arrows show the increas-ing/decreasing of the temperature. At low temperature the irreversibility is clearly observed. Theinsert displays the rapid increase of the susceptibility in the FC mode, occurring at about 12 K. used to determine the magnetic characteristics. As already mentioned, at high temperaturethe compound follows a Curie-Weiss law that was fitted with a constant, C = 2 . ± . cm K/mol , and an extrapolated Curie-Weiss temperature θ CW from −
19 to −
24 K. This Cvalue is well fitted with a total number of Co spins contribution s = 3 /
2, and a small orbitalangular contribution l <
1. It is important to mention that in orthorhombic crystals (also incubic crystals) structures the orbital moment vanishes at first order. In this case the smalldeformation of the orthorhombic structure (the angles are not exactly 90 degrees) gives thepossibility, as we observed, that the orbital angular moment l is not totally quenched. Inthe top insert of this figure we plotted an amplification of the inverse susceptibility from2 K to 20 K. At about 12.7 K an abrupt change of slope occurs. In the inferior insert wedisplay the effective number of Bohr magnetons, µ B , at room temperature are equal to 4.3 µ B . In the same figure and at low temperature a ferrimagnetic transition can be deduced;this ferrimagnetic ordering is according to the negative θ CW values obtained by the fitting.nevertheless, as a confirmation of the ferrimagnetic behavior, and the crossover to highspin by influence of the magnetic intensity, we studied the isothermal magnetic M − H characteristics at different temperatures, from 2 K to 20 K. The coercive field already isobserved, and it is shown in the main panel of 4. It is clear that the coercive field tends todisappear at temperatures of the order of 15 K. The insert in this figure also presents one8 - ( m o l / c m ) T (K) T= 12.7 K e ff ( B M ) T (K) - ( m o l / c m ) T (K)
FIG. 3. (Color online) Inverse of the magnetic susceptibility χ − ( T ) measured in ZFC and FCmodes, at 100 Oe. The continuous line in the main panel and the top insert shows the fit to theCurie-Weiss law. Curie constant value was determined to be C = 2 . ± . cm K/mol, and θ CW changes from 19 - 24 K. The top insert shows an amplification of the main panel at lowtemperature. There, the vertical arrow marks the change of the slope, which is about 12.1 K. Theinferior insert displays the effective number of Bohr magnetons, determined as 4.25 µ B at roomtemperature. H C ( O e ) Temperature (K) M ( - e m u ) H (kOe)
T= 2.6 K
FIG. 4. (Color online) Coercive field determined by isothermal M − H measurements at differ-ent temperatures. The ferromagnetic ordering persists only around 15 - 20 K. The insert showsthe influence of magnetic field in the magnetic properties, This small spin crossover indicates ametamagnetic transition changing the magnetic ordering from ferrimagnetic to ferromagnetic.
50 100 150 200 250 300020406080100120140 - ( m o l / c m ) T (K)
C=2.43 Kcm /mol s =3/2; l <1 CW = -19 KH=1000 Oe. ( c m / m o l ) Temperature (K)
FCZFC
FIG. 5. (Color online) Main panel presents the magnetic susceptibility, χ ( T ), measured with 1000Oe, from 30 K to 2 K. The characteristic shows important differences of behavior that are stronglydependent on the intensity of the magnetic field applied. At higher field, 5000 Oe, the behavior iscompletely reversible, as noted by completed reversibility. This behavior marks the metamagnetictransition, by influence of the applied magnetic field. isothermal M − H at 2.6 K to illustrate crossover from low spin to high spin.5 displays the susceptibility as a function of temperature measurements into magneticfields of 1000 Oe and 5000 Oe. Note that the irreversible temperature changes at lowertemperature and disappears at 5000 Oe. We interpreted this behavior as a metamagnetictransition. However in order to have a clear confirmation of this feature we plotted at lowtemperature isothermal M − H measurements. This characteristic shows a small but dis-cernible change from low to high spin as the magnetic intensity is increased. Metamagnetismis due to the applied field strength that overcome the crystal anisotropic force, producingan abrupt change in the internal order [21]. C. Specific heat
Lastly, in order to confirm the complicated physical characteristics of this boracite, westudied the specific heat at low temperatures. Our experiments show an anomalous widepeak, that we characterized as a Schottky anomaly (6). This occurs at low temperatures,where spin population may be excited by thermal energy between e g and t g levels of thesystem. This Schottky anomaly occurs at about 4.3 K, the thermal energy at this tempera-ture is about 370 × − eV, corresponding approximately to the splitting of the ± s = 3 / Temperature (K)
H = 0 Oe S pe c i f i c H ea t ( J / m o l e K ) H = 100 Oe
H = 0.5 T
FIG. 6. (Color online) Specific heat at low temperature of the boracite Zn . Co . B O Br mea-sured at magnetic field intensities of 0, 100, and 5000 Oe. The feature displayed at about 4.5 K,clearly can be catalogued as a Schottky anomaly. states.
IV. CONCLUSIONS
A new Zn . Co . B O Br boracite was synthesized by the vapor transport method. Thecompound has a orthorhombic crystal structure, with small distortions, and with the spacegroup Pca2 (No. 29) and cell parameters a = 8.5705(3), b = 8.5629(3), and c = 12.1198(4) ˚A,and Volume = 889.45(3) ˚A . Magnetic measurements performed as a function of temperatureand at different magnetic intensities show that the general behavior at low temperaturechanges; first at all, the irreversibility disappears at an intensity of about 5000 Oe, andthe anomalies change. This behavior is because the low to high spins crossover, affectingthe general characteristics. We identify this reduction of irreversibility as a metamagnetictransition. Lastly, specific heat measurements at low temperature show an anomaly thatclearly was identified as a Schottky anomaly.11 CKNOWLEDGMENTS
RE thanks CONACyT project 129293, and DGAPA UNAM project IN100711. FMthanks the partial support of DGAPA UNAM project IN111511. We also thank to R. Reyesand to F. Silvar for help in technical problems. [1] Levasseur, A.
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