Sandwich-like Layer Research Articles

Constructed 2D sandwich-like layer WO3/Ti3C2/ZnIn2S4 Z-scheme heterojunction by chemical bond for effective photocatalytic hydrogen production

Photocatalytic water-splitting has gained significant global attention in recent years. However, identifying effective photocatalysts remains challenging due to the rapid recombination of photoinduced charge carriers. In this study, two-dimensional (2D) sandwich-like layer WO3/Ti3C2/ZnIn2S4 photocatalysts were successfully fabricated using a simple anaerobic solvothermal process. The 2D Z-scheme heterojunction enhances rapid charge transport via TiS or TiOW bonds, serving as efficient charge transfer channels and minimizing the distance for interfacial photocarrier transfer. Consequently, the hydrogen production rate of 20% WO3/Ti3C2/ZnIn2S4 composite reaches 7.39mmol·g-1·h-1, which is 3.5 and 7.1 times higher than that of 20% Ti3C2/ZnIn2S4 and pure ZnIn2S4, respectively. Furthermore, the hydrogen production rate of 20% WO3/Ti3C2/ZnIn2S4 composite reaches 2.54mmol·g-1·h-1 without the use of sacrificial agents. This work paves the way for designing 2D sandwich-like Z-scheme heterostructures through interfacial chemical bonds.

Heteroatom-Doped Molybdenum Disulfide Nanomaterials for Gas Sensors, Alkali Metal-Ion Batteries and Supercapacitors.

Molybdenum disulfide (MoS2) is the second two-dimensional material after graphene that received a lot of attention from the research community. Strong S-Mo-S bonds make the sandwich-like layer mechanically and chemically stable, while the abundance of precursors and several developed synthesis methods allow obtaining various MoS2 architectures, including those in combinations with a carbon component. Doping of MoS2 with heteroatom substituents can occur by replacing Mo and S with other cations and anions. This creates active sites on the basal plane, which is important for the adsorption of reactive species. Adsorption is a key step in the gas detection and electrochemical energy storage processes discussed in this review. The literature data were analyzed in the light of the influence of a substitutional heteroatom on the interaction of MoS2 with gas molecules and electrolyte ions. Theory predicts that the binding energy of molecules to a MoS2 surface increases in the presence of heteroatoms, and experiments showed that such surfaces are more sensitive to certain gases. The best electrochemical performance of MoS2-based nanomaterials is usually achieved by including foreign metals. Heteroatoms improve the electrical conductivity of MoS2, which is a semiconductor in a thermodynamically stable hexagonal form, increase the distance between layers, and cause lattice deformation and electronic density redistribution. An analysis of literature data showed that co-doping with various elements is most attractive for improving the performance of MoS2 in sensor and electrochemical applications. This is the first comprehensive review on the influence of foreign elements inserted into MoS2 lattice on the performance of a nanomaterial in chemiresistive gas sensors, lithium-, sodium-, and potassium-ion batteries, and supercapacitors. The collected data can serve as a guide to determine which elements and combinations of elements can be used to obtain a MoS2-based nanomaterial with the properties required for a particular application.

High-Performance Dual-Ion Battery Based on a Layered Tin Disulfide Anode.

Energy issues have attracted great concern worldwide. Developing new energy has been the main choice, and the exploitation of the electrochemical energy storage devices plays an important role. Herein, a high-performance dual-ion battery system is proposed, which consists of a graphite cathode and SnS2 anode, with a high-concentration lithium salt electrolyte (4 M LiTFSI). The benefits from the typical sandwich-like layer structure of SnS2 are as follows: the highest discharge specific capacity of the battery could reach 130.0 mA h g–1 at a current density of 100 mA g–1, and even under an ultra-high current density of 2000 mA g–1, the highest capacity of 66.3 mA h g–1 is still achieved, with an outstanding capacity retention over 100% after 1000 cycles. Inspiringly, this system delivers an excellent low self-discharge of 1.19%/h, surpassing most of the reported dual-ion batteries. In addition, the working mechanism and structural stability are also investigated by X-ray diffraction and Raman spectra, indicating a good reversibility. These results reveal that this graphite/SnS2 dual-ion battery system could provide a promising alternative for a future high-performance energy storage device.

Micro-band Boron-doped Diamond Electrode in Capillary Electrophoresis for Simultaneous Detection of AMP, ADP, and ATP

A micro-band boron-doped diamond (BDD) electrode was prepared by sealing a piece of BDD film with an area of 1.11´10-7 m2 between two insulating plates, one Teflon and one silicon rubber, to form sandwich-like layers, so the surface area could b

Crystal structures of four inclusion compounds of 4,4’-dihydroxy diphenyl sulfide and tetraalkylammonium

Four novel inclusion compounds of (C2H5)4N+·C12H9O2S-·2H2O (1), (n-C3H7)4N+·C12H9O2S- (2), (n-C3H7)4N+·C12H9O2S-·C12H10O2S (3) and (n-C4H9)4N+·C12H8O2S2-·CH5N3 (4) were prepared by 4, 4’-dihydroxy diphenyl sulfide and various tetraalkylammonium guest templates. Crystal structures of four inclusion compounds are characterized by single crystal X-ray diffraction, Among them, 1 and three belong to orthorhombic crystal system with different space groups of P212121 and Pca21, while 2 and four are monoclinic system with P21/c and C2/c. In addition, 2 and three are polymorphs. The crystallography data are shown as following: compound 1: a = 6.9675(8) Å, b = 15.4889(16) Å, c = 20.074(2) Å, V = 2166.36(4) Å3, Z = 4; compound 2: a = 8.8441(3) Å, b = 17.8886(4) Å, c = 15.8642(3) Å, V = 2422.18(11) Å3, Z = 4; compound 3: a = 16.6500(3) Å, b = 11.5233(2) Å, c = 18.1212(3) Å, V = 3476.79(10) Å3, Z = 4; compound 4: a = 9.7001(16) Å, b = 21.007(3) Å, c = 25.138(4) Å, V = 5122.03(14) Å3, Z = 8. It is worth noting that the host molecules of 4, 4’-dihydroxydiphenyl sulfide in the four conclusion compounds, with or without assistant small molecules as the proton donors, generate varied host lattices, and the similar guest templates of tetraalkylammonium cations are filled in the cavities of the host lattices to produce peanut-shape network structure, wave-like layer structure, adduct crystal structure and sandwich-like layer structure respectively. Obviously, the presences of auxiliary molecules have great influences on the formation of the final crystal structures.

Similar hydrogen-bonded ribbons in three 4,4’-sulfonyldibenzoic anion/boric acid and tetrabutylammonium inclusion compounds

Three novel inclusion compounds of 4,4’-sulfonyldibenzoic anion, boric acid and tetrabutylammonium, 2[(C4H9)4N]+·[C14O6H8S]2−·3.5[B(OH)3]·1.5H2O (1), 2[(C4H9)4N]+·[C14O6H8S]2−·2[B(OH)3]·7H2O (2) and 1.5[(C4H9)4N]+·0.5[C14O6H9S]2−·0.5[C14O6H9S]−·2[B(OH)3] (3), were prepared and characterized by single crystal X-ray diffraction, IR spectroscopy and thermogravimetric analyses (TGA). Compound 1: triclinic system, P¯ 1, a = 9.371(3) Å, b = 17.624(5) Å, c = 18.678(6) Å, α = 92.471(3)°, β = 101.837(4)°, γ = 91.331(4)°, V = 3014.9(15) Å3, Z = 2, R1 = 0.0902; compound 2: orthorhombic, Pca21, a = 19.8104 Å, b = 9.2340 Å, c = 34.9875 Å, V = 6400.2(18) Å3, Z = 4, R1 = 0.0728; compound 3: triclinic, P¯ 1, a = 10.0088(1) Å, b = 13.2179(1) Å, c = 18.945(2) Å, α = 70.6850(1)°, β = 86.9100(1)°, γ = 71.3400(1)°, V = 2237.2(4) Å3, Z = 2, R1 = 0.0628. The results show that the host 4, 4’-sulfonyldibenzoic anion can combine with boric acid to form the similar hydrogen-bonded ribbons through O-H…O interactions in these three compounds. However, with the existence of the same guest template of tetrabutylammonium, the similar ribbons finally generate two sandwich-like layer frameworks of 1 and 2, and one 3D network containing cavities of 3. Clearly, the participation of water molecules should be mainly responsible for the distinct difference between the layer structure and the 3D network.

Fabrication of biomolecules coated nanostructured oxide layer to facilitate cell adhesion and proliferation for improving osseointegration

Abstract This study was to elucidate the surface characteristics and in vitro cell behaviors of biomedical titanium (Ti) with the biofunctional surface (nanostructured rutile-Ti dioxide (NanoR-TiO2) layer combined with amino-groups (NH2) and albumin). Biofunctional surface features were analyzed via scanning electron microscopy, atomic force microscopy, and transmission electron microscopy. Immunolabeling with nano-gold particles was used to reflect the expected specific binding of protein on the biofunctional surface. In vitro cell behaviors were evaluated by culturing with mouse pre-osteoblastic cell line (MC3T3-E1) at 37 °C in different periods. Analytical results indicated that the sandwich-like (NH2/NanoR-TiO2/Ti) layer was formed on the Ti surface after plasma oxidation and polymerization. Immunolabeling analysis results also proved that the albumin can be successfully bonded with NH2 on NanoR-TiO2 layer. Moreover, cell morphology observation revealed that the biofunctional surface could potentially facilitate cell adhesion and proliferation. Therefore, the biomedical Ti with the biofunctional surface is a promising biomaterial for dental implant applications.

Enzyme-Modulated Anaerobic Encapsulation of Chlorella Cells Allows Switching from O2 to H2 Production.

Single-cell encapsulation has become an effective strategy in cell surface engineering; however, the construction of cell wall-like layers that allow the switching of the inherent functionality of the engineered cell is still rare. In this study, we show a universal way to create an enzyme-modulated oxygen-consuming sandwich-like layer by using polydopamine, laccase, and tannic acid as building blocks, which then could generate an anaerobic microenvironment around the cell. This layer protected the encapsulated C. pyrenoidosa cell against external stresses and enabled it to switch from normal photosynthetic O2 production to photobiological H2 production. The layer showed an smaller effect on the PSII activity, which contributed a significant enhancement on the rate (0.32 μmol H2 h-1 (mg chlorophyll)-1 ) and the duration (7 d) of H2 production. This strategy is expected to provide a pathway for modulating the functionality of cells and for breakthroughs in the development of green energy alternatives.

Atmospheric Layers in Response to the Propagation of Gravity Waves under Nonisothermal, Wind-shear, and Dissipative Conditions

We study the atmospheric structure in response to the propagation of gravity waves under nonisothermal (nonzero vertical temperature gradient), wind-shear (nonzero vertical zonal/meridional wind speed gradients), and dissipative (nonzero molecular viscosity and thermal conduction) conditions. As an alternative to the “complex wave-frequency” model proposed by Vadas and Fritts, we employ the traditional “complex vertical wave-number” approach to solving an eighth-order complex polynomial dispersion equation. The empirical neutral atmospheric models of NRLMSISE-00 and HWM93 are employed to provide mean-field properties. In response to the propagation of gravity waves, the atmosphere is driven into three sandwich-like layers: the adiabatic layer (0–130 km), the dissipation layer (130–230 km) and the pseudo-adiabatic layer (above 230 km). In the lower layer, (extended-)Hines’ mode or ordinary dissipative wave modes exist, whereas viscous dissipation and thermal conduction fail to exert perceptible influences; in the middle layer, Hines’ mode ceases to exist, and both ordinary and extraordinary dissipative wave modes flourish; in the top layer, only extraordinary wave modes survive, and dissipations affect the real part of the vertical wavenumber ( m r ) substantially; however, they contribute little to the imaginary part, which is the vertical growth rate ( m i ). We also analyze the transition of Hines’ classical mode to ordinary dissipative wave modes, describe both the upward and downward modes of gravity waves and illustrate nonisothermal and wind-shear effects on the propagation of gravity waves of different modes.

N-H···S and C-H···S hydrogen bonds in two tetraalkylammonium dithiobiurea(1-) inclusion compounds.

Two inclusion compounds of dithiobiurea and tetrapropylammonium and tetrabutylammonium are characterized and reported, namely tetrapropylammonium carbamothioyl(carbamothioylamino)azanide, C12H28N(+)·C2H5N4S2(-), (1), and tetrabutylammonium carbamothioyl(carbamothioylamino)azanide, C16H36N(+)·C2H5N4S2(-), (2). The results show that in (1), the dithiobiurea anion forms a dimer via N-H···N hydrogen bonds and the dimers are connected into wide hydrogen-bonded ribbons. The guest tetrapropylammonium cation changes its character to become the host molecule, generating pseudo-channels containing the aforementioned ribbons by C-H···S contacts, yielding the three-dimensional network structure. In comparison, in (2), the dithiobiurea anions are linked via N-H···S interactions, producing one-dimensional chains which pack to generate two-dimensional hydrogen-bonded layers. These layers accommodate the guest tetrabutylammonium cations, resulting in a sandwich-like layer structure with host-guest C-H···S contacts.

2-(1-Amino-4-tert-butylcyclohexyl)acetic acid (tBu-β3,3-Ac6c) hemihydrate

The title compound, C12H23NO2·0.5H2O, crystallized with two 2-(1-amino-4-tert-butylcyclohexyl)acetic acid mol­ecules, which are present as zwitterions, and one water mol­ecule in the asymmetric unit. The mol­ecular structure of each zwitterion is stabilized by an intra­molecular six-membered (C 6 ) N—H⋯O hydrogen bond. In the crystal, the two independent zwitterions are linked head-to-head by N—H⋯O hydrogen bonds. Further O—H⋯O and N—H⋯O hydrogen bonds link the zwitterions and the water molecules, forming sandwich-like layers, with a hydrophilic filling and a hydrophobic exterior, lying parallel to the ab plane.

Hydrothermal synthesis, crystal structure, and photoluminescence of a new 2D cadmium(II) complex: {Cd(C8H5O3Cl2)2(C10H8N2)} n

One new cadmium coordination polymer, [Cd(2,4-Dcp)2(4,4-Bipy)]n (I) (2,4-HDcp = 2,4-dichlorophenoxyacetic acid, 4,4′-Bipy = 4,4′-bipyridine) with 2D layer structure, has been prepared by the hydrothermal synthesis and characterized by elemental analysis, IR, TGA and single-crystal X-ray diffraction. Complex I crystallizes belong to monoclinic system and has C2/c space group. Each Cd2+ ion is six-coordinated and located at an octahedral geometry. The Cd2+ ions are linked by bidentate 2,4-Dcp groups into a linear chain in which the benzene rings of 2,4-Dcp ligands point alternately up and down. These chains are further connected into a sandwich-like layer though 4,4′-Bipy ligands. Furthermore, the photoluminescence and life-time of I in the solid state have been studied.

The T′ phase and its ‘sandwich-like layer’ structure as shown by ionic liquid crystals containing a biphenyl ester-based rigid-core modified by 3-alkylimidazolium salts

Eight ionic liquid crystals containing a biphenyl ester-based rigid core modified by a 3-alkyl-imidazolium cation were synthesised and characterised. The length of the alkyl chain varied between n = 4–18 carbon atoms. Their thermotropic phase behaviour was investigated using X-ray diffraction (XRD) measurements, variable temperature small-angle X-ray scattering (SAXS), polarising optical microscopy (POM) and differential scanning calorimetry (DSC). The compounds show rich mesomorphism, such as SmA2 phase (for compound C18) and SmC2 phases (for compounds C12–C16). For the molecules containing long alkyl chains (n ≥ 12), a novel mesophase, the T′ phase, was seen which has a unique ‘sandwich-like layer’ structure. The T′ phase is different from the known T phase and is formed at room temperature. Based on XRD, SAXS, DSC and POM data, models for the formation of the T′ phase are proposed.

A novel l-lactate sensor based on enzyme electrode modified with ZnO nanoparticles and multiwall carbon nanotubes

Abstract A highly sensitive and stable l -lactate sensor based on the synergic action of multi-wall carbon nanotube (MWCNTs) and ZnO nanoparticles has been developed. The unique sandwich-like layer structure (PDDA/LOD/ZnO/MWCNTs) provides a favorable microenvironment to keep the bioactivity of LOD and prevent enzyme molecule leakage. Therefore, the proposed l -lactate biosensor exhibited good analytical performances including high sensitivity and selectivity with satisfactory stability to amperometric determination of l -lactate. The results indicated that the proposed PDDA/LOD/ZnO/MWCNTs film was an attractive matrix for the immobilization of LOD enzymes to fabricate biosensors.

Multilayered ordered mesoporous platinum/titania composite films: does the photocatalytic activity benefit from the film thickness?

Multilayered films of TiO2 with ordered cubic mesoporosity were grown via layer-by-layer deposition on a conductive FTO (F-doped SnO2) substrate by dip-coating and subsequent calcination at 400 °C. Since platinum nanoparticles are known to enhance the photocatalytic activity, they were introduced into the TiO2 mesopores by pulsed electrodeposition. Additionally, sandwich-like layers with up to five alternating TiO2 and Pt layers were prepared. The photocatalytic gas-phase oxidation of acetaldehyde served as a test reaction to characterize the activity in the gas phase of both pristine TiO2 as well as Pt/TiO2 single- and multilayer films. The ordered mesoporous pristine TiO2 and Pt/TiO2 nanocomposites exhibited significantly higher photoactivity than commercial Pilkington Activ™ glass and dense TiO2 films. Moreover for pristine TiO2 films, those consisting of three layers (about 650 nm in thickness), were shown to be sufficient to achieve a maximum photonic efficiency of ζ = 0.45%. For the Pt/TiO2 system, however, a single-layer film with a total thickness of only about 220 nm exhibited an almost identical activity. Moreover, repetitive experiments demonstrated that the newly prepared photocatalyst films did not suffer from a decrease in the photocatalytic activity, evincing their potential for practical applications.

Einige Phosphidhalogenide des Lanthans und verwandte Verbindungen/ Some Phosphide Halides of Lanthanum and Related Compounds

The pnictide halides La2I2P, La2I2As, La2I2Sb, La2Br2P and Y2Br2P have been synthesized from lanthanum and yttrium, red phosphorus, arsenic, and antimony, respectively, and the corresponding metal trihalides. Their structures contain close-packed metal atom double layers with pnicogen atoms in the octahedral voids. These layers are sandwiched by halogen atom layers. The compounds crystallize in the trigonal 1T-type with one sandwich-like layer per unit cell, or in the rhombohedral 3R-type with three layers per unit cell. Polytypism and twinning have been observed. For 3R-La2I2P, conductivity measurements have shown metallic behaviour

Amperometric glucose biosensor based on self-assembling glucose oxidase on carbon nanotubes

A flow injection amperometric glucose biosensor based on electrostatic self-assembling glucose oxidase (GOx) on a carbon nanotube (CNT)-modified glassy carbon transducer is described. GOx is immobilized on the negatively charged CNT surface by alternatively assembling a cationic polydiallyldimethylammonium chloride (PDDA) layer and a GOx layer. The unique sandwich-like layer structure (PDDA/GOx/PDDA/CNT) formed by self-assembling provides a favorable microenvironment to keep the bioactivity of GOx and to prevent enzyme molecule leakage. The direct electrochemistry behavior of GOx and electrocatalysis of H2O2 on the fabricated PDDA/GOx/PDDA/CNT electrode demonstrated that such a biosensor fabrication method preserves the activity of enzyme molecules and the mechanical and electrocatalytic properties of carbon nanotubes, enabling sensitive determination of glucose. Flow injection amperometric detection of glucose is carried out at -100 mV (vs Ag/AgCl) in 0.05 M phosphate buffer solution (pH 7.4) with wide linear response range of 15 uM- 6 mM and a detection limit of 7 uM. The PDDA/GOx/PDDA/CNT/GC biosensor showed excellent properties for the sensitive determination of glucose with good reproducibility, remarkable stability, and free of interference from other co-existing electroactive species. The present methods can be applied to assemble other enzyme molecules and biological molecules, such as antibody, antigen, and DNA, to the CNTmore » surface for wide biosensor and bioassay applications.« less

Crystal and molecular structure of 1-piperidiniumacetic acid chloride studied by X-ray, B3LYP and PM3 calculations

Abstract The 1:1 complex of 1-piperidineacetic acid (PAA) with HCl has been studied by X-ray diffraction and by B3LYP/6-31G(d,p) and PM3 calculations. The crystals are monoclinic, space group P 2 1 / c , a =13.268(1), b =6.349(1), c =11.083(1) A, β =99.79(1)°, Z =4, R =0.0372. The carboxylate group of PAA is protonated and forms the O–H⋯Cl − hydrogen bond with chlorine anion (2.964(1) A). The chlorine anion is also engaged in the N + –H⋯Cl − hydrogen bond (3.062(1) A) and C–H⋯Cl − contacts (3.552(2) and 3.784(2) A), consolidating the layers forming their hydrophilic internal areas. On both sides of the layers there are hydrophobic piperidinium residues having only van der Waals contacts with the next sandwich-like layers. In the optimised by the B3LYP/6-31G(d,p) calculations two protonated molecules of PAA are joined by the O–H⋯Cl − and N + –H⋯Cl − hydrogen bonds forming the cyclic dimer. In the structure predicted by the PM3 method the neutral PAA molecule is engaged in the N⋯H–Cl hydrogen bond, two such molecules are connected by one O–H⋯Cl hydrogen bond forming an open dimer. The molecular structures of complexes of simple amino acids with mineral acids are discussed.

Temperatures in Grinding of Magnetic Composites - Theoretical and Experimental Approach

The present paper is concerned with grinding temperatures and surface Integrity for grinding of magnetic composites consisting of sandwich-like layers of brittle magnetic material and soft steel. High temperatures which are generated during grinding of these magnetic composites may have a detrimental effect on their magnetic properties if the grinding temperature exceeds the Curie point of the magnetic component. A thermal model was developed to calculate the grinding temperatures in the magnetic composite workplace. Temperatures measured in the magnetic composite using embedded thermocouples were found to be consistent with predictions from the thermal model. Crucial regions in the magnetic composite were found where the highest temperatures and temperature gradients occur and where surface integrity may be destroyed. The range of grinding conditions providing both high efficiency and good surface quality were found.

Clinotobermorite, Ca5[Si3O8(OH)]2 · 4 H2O \p=n-\ Ca5[Si6O17] · 5 H2O, a natural C\p=n-\S\p=n-\H(I) type cement mineral: determination of the substructure

Abstract The substructure of natural clinotobermorite Ca5[Si3O8(OH)]2 · 4 H2O \p=n-\ Ca5[Si6Ol7] · 5 H2O from the Wessels mine, Kalahari manganese field, South Africa, was determined from single-crystal X-ray data. The diffraction pattern (monoclinic: a = 11.19 Å, b = 7.29 Å, c = 22.46 Å, β = 96.97°) is characterised by rather diffuse reflections of the type hkl with h, k = 2n + 1. The substructure (space group I2/m, Z = 1, a/2 = 5.593(6) Å, b/2 = 3.645(4) Å, c = 22.46(3) Å, β = 96.97(2)°) was solved from the sharp Bragg reflections (hkl, with h, k = 2n) and refined on the basis of 1877 unique reflections to R(F) = 6.7%. The structure of clinotobermorite is closely related to the one of so called 11 Å tobermorite Ca5Si6O16(OH)2 · 2 H2O (pseudo-orthorhombic, a = 11.17 Å, b = 7.38 Å, c = 22.6 Å). In both minerals layers parallel (001), formed by seven-fold co-ordinated Ca, are flanked on either side by wollastonite-like [Si3O9] single chains (ca. 7.3 Å periodicity along b) creating a sandwich-like assemblage. The short b/2 axis (3.645 Å) and the short a/2 axis (5.593 Å) of the subcell are caused by an overlay of two half-occupied single chains of tetrahedra with one shifted by b/2 against the other. In clinotobermorite the sandwich-like layers are stacked along c yielding an interlayer distance of 4.5 Å with a free aperture of 3.2 Å where additional Ca ions and H2O molecules connect the sandwich units. Clinotobermorite with 4–5 H2O represents a much denser structure than 11 Å tobermorite with only 2 H2O or 14 Å tobermorite with ca. 6 H2O (pseudo-orthorhombic, a = 11.17 Å, b = 7.38 Å, c = 27.94 Å). The substructure of clinotobermorite can be interpreted as either composed of ‘Dreiereinfachketten’ leading to the formula Ca5[Si3O8(OH)]2 · 4 H2O, or of ‘Dreierdoppelketten’ leading to the formula Ca5[Si6O17] · 5 H2O, or of a mixture of both variants. The surface structure of clinotobermorite was studied by atomic force microscopy (AFM) where low magnifications reveal a complex layering or twinning parallel (001). High resolutions scans parallel (100) establish enhanced contrast fringes with a periodicity of ca. 10 Å interpreted as the repeat distance of the Ca polyhedral layers parallel to c.