New Family of Materials with Negative Coefficients of Thermal

Article pubs.acs.org/IC

New Family of Materials with Negative Coefficients of Thermal Expansion: The Effect of MgO, CoO, MnO, NiO, or CuO on the Phase Stability and Thermal Expansion of Solid Solution Phases Derived from BaZn2Si2O7 Christian Thieme,* Tina Waurischk, Stephan Heitmann, and Christian Rüssel Otto-Schott-Institut für Materialforschung, Jena University, Fraunhoferstr. 6, 07743 Jena, Germany ABSTRACT: Recently, a silicate with the composition SrxBa1−xZn2Si2O7 was reported, which exhibits a negative coefficient of thermal expansion. The compound BaZn2Si2O7 shows a highly positive coefficient of thermal expansion up to a temperature of 280 °C and then transfers to a high temperature phase, which exhibits a coefficient of thermal expansion near zero or negative over a limited temperature range up to around 500 °C. This high temperature modification can be stabilized to room temperature if Ba2+ is replaced by Sr2+. In the solid solution SrxBa1−xZn2Si2O7, also Zn2+ can be replaced in a wide concentration range by other cations with the respective valency. In the present study, Zn was partially or completely replaced by Mg, Co, Mn, Ni, or Cu. If the high temperature phase is stable at room temperature, the thermal expansion is negative, and if the partial substitution exceeds a certain concentration threshold, the low temperature phase with the crystal structure of BaZn2Si2O7 and highly positive thermal expansion is formed. The lowest mean coefficients of thermal expansion were measured for the composition Ba0.5Sr0.5Zn1.4Co0.6Si2O7 with a value of −2.9 × 10−6 K−1. In general, a lower Zn-concentration leads to a higher anisotropy and a lower mean coefficient of thermal expansion.

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INTRODUCTION Materials with a coefficient of thermal expansion (CTE) close to zero are widely used in different fields of application.1,2 They offer challenging properties such as excellent thermal shock resistance, and their dimension and shape are not affected by varying temperatures.3 One of the most important applications are cooktop panels and large size telescope mirrors for astronomy as well as numerous devices in micromechanics.4,5 In the case of glass-ceramics, a thermal expansion close to zero is commonly achieved by a material in which at least one phase has a negative thermal expansion coefficient.6 At the moment, commercial materials with ultralow thermal expansion are exclusively made from glasses in the base system Li2O−Al2O3− SiO2 with various additives, such as ZnO, MgO, TiO2, and ZrO2.7−10 The latter two are used as nucleating agents. Glasses, ceramics, and glass ceramics based on silicates with high concentrations of alkaline earth ions normally exhibit very high CTEs, caused by high CTE phases such as barium silicates.11−15 In the case of BaZn2Si2O7, two phases are reported: an orthorhombic high temperature phase (HT-phase) and a monoclinic low temperature phase (LT-phase) with a phase transition temperature at around 280 °C.16 Both phases exhibit strongly different thermal expansion characteristics: i.e., BaZn2Si2O7 shows high thermal expansion below 280 °C, and above this temperature, low and partly negative thermal expansion occurs.14 In the case of the LT-phase, the Zn2+© XXXX American Chemical Society

ions are arranged in chains and they are in 4- and 5-fold coordination. A detailed description of BaZn2Si2O7 and isostructural compounds can be found in refs 16−18. Recently, a new phase in the solid solution series SrxBa1−xZn2Si2O7 with a crystal structure similar to that of HT-BaZn2Si2O7 was reported.19 The crystal structure of this compound is orthorhombic, and the space group is Cmcm.19 The Zn2+-ions are incorporated into tetrahedra; i.e., they are surrounded by four oxygen ions. These tetrahedra form chains, which run through the crystal in a direction parallel to the crystallographic c-axis, and the chains are connected via Si2O7-units. In between, the alkaline earth ions can be found, which are in a 5-fold coordination. The phase exhibits strongly anisotropic and negative thermal expansion. As it is known for BaZn2Si2O7, solid solutions with divalent ions exist in a broad concentration range leading to a shift of the stability range of the two phases.12 If ZnO is fully replaced by MgO14 or CoO15 or partially by NiO,12 then the temperature attributed to the phase transition increases steadily with the respective concentrations. The thermal expansion is then highly positive in the entire temperature range until the phase transition temperature is reached. Depending on the particular chemical composition, the coefficient of thermal expansion can be higher than 15 × Received: February 3, 2016

A

DOI: 10.1021/acs.inorgchem.6b00290 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry 10−6 K−1 13 and the phase transition temperature can be shifted to temperatures above 1000 °C.20 The increase in volume, which correlates with the phase transition, decreases if ZnO is partially or fully replaced by other divalent ions with similar ionic radii.14,15 It should be mentioned that the phase BaZn2Si2O7 and also solid solutions with the compositions BaZn2−xMxSi2O7 (M = Co, Ni) have already been crystallized from typical sealing glasses by an appropriate thermal treatment.11,20 The as prepared glasses can be utilized as crystallizing seals with high CTEs for high-temperature applications. The introduction of SrO in such sealing glass ceramics should strongly affect the thermal expansion by reducing the CTE of the seal to undesired low values caused by the formation of SrxBa1−xZn2Si2O7 solid solutions. However, glass ceramics with high volume concentrations of the as mentioned solid solutions could be an alternative material for commercial zero thermal expansion glass ceramics. Therefore, the phase stability of solid solutions as well as the thermal expansion characteristics have to be determined. In this paper, the effect of Zn substitution by Mg, Co, Ni, Cu, Fe, and Mn was studied with respect to the phase stability. Xray diffraction was used to study the appearing phases at room temperature as well as the thermal expansion at temperatures up to 1000 °C.

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Table 1. Synthesis Temperatures and Occurring Phases (HT = High Temperature, LT = Low Temperature Phase) of Ba0.5Sr0.5Zn2−xMxSi2O7 Solid Solutions with M = Mg, Co, Mn, Ni, and Cu and Different Values of x

EXPERIMENTAL PROCEDURE

The samples were synthesized by a solid state reaction from mixed oxides and carbonates. SiO2 (Carl Roth GmbH + Co. KG, >99%), BaCO3 (VK Labor- and Feinchemikalien, pure), SrCO3 (Ferak Berlin, >99%), ZnO (Carl Roth, >99%), MgO (Merck, heavy extra pure), MnCO3 (VEB Laborchemie Apolda, pure), Co3O4 (VEB Laborchemie Apolda, pure), NiO (HB Labor- and Feinchemikalien, pure), CuO (HB Labor- and Feinchemikalien, extra pure), and Fe2O3 (SigmaAldrich, ≥99%) were used as raw materials. The mixtures were heated up to temperatures in the range from 1100 to 1300 °C and kept for 30 to 50 h with several intermediate regrinding steps. A more detailed overview of the chosen synthesis temperatures is given in Table 1. S am pl e s w e r e p r e p ar e d i n th e so l i d so l u t i o n se r i e s Ba0.5Sr0.5Zn2−xMxSi2O7 (M = Mg, Co, Mn, Ni, Cu) with x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, and 0.7. However, the maximum value of x differs for the elements M; i.e., at the highest x-value, a mixture of the HTand LT-phase was found. In order to control that no volatile species such as ZnO have been evaporated, the samples were weighed after the thermal treatments. Furthermore, the phase purity was checked via X-ray powder diffraction (XRD) using a SIEMENS D5000 Bragg-Brentano diffractometer and Cu Kα radiation. The XRD-patterns of the powdered samples were recorded in a 2θ-range from 10° to 60° with a step size of Δθ = 0.02° and a counting time of 1−3 s per step depending on the signal-to-noise ratio of the samples. The lattice parameters were obtained using the software TOPAS 3 from BRUKER and the whole powder pattern decomposition method (Pawley method). The starting values of the refinement were taken from ref 19. This procedure was also chosen to obtain the lattice parameters at high temperatures, i.e., the thermal expansion behavior up to 1000 °C. Therefore, the as mentioned X-ray diffractometer was equipped with an ANTON PAAR HTK 10 heating stage. The samples were mixed with α-Al2O3 as an internal standard with known thermal expansion.21 This was necessary in order to correct the sample displacement caused by the thermal expansion of the sample holder. The powdered samples were also measured in a 2θ-range from 10° to 60° with a step size of Δθ = 0.02°. The counting time was 1 s. After heating the samples to the respective measuring temperature with 5 K/s, the temperature was kept for several minutes until the temperature stabilized. Afterward, the samples were measured with the as mentioned parameters. The accuracy of the coefficients of thermal expansion determined with this

composition

sample name

synthesis temperature [°C]

Ba0.5Sr0.5Zn2Si2O7 Ba0.5Sr0.5Zn1.9Mg0.1Si2O7 Ba0.5Sr0.5Zn1.8Mg0.2Si2O7 Ba0.5Sr0.5Zn1.7Mg0.3Si2O7 Ba0.5Sr0.5Zn1.6Mg0.4Si2O7 Ba0.5Sr0.5Zn1.5Mg0.5Si2O7 Ba0.5Sr0.5Zn1.4Mg0.6Si2O7 Ba0.5Sr0.5Zn1.3Mg0.7Si2O7 Ba0.5Sr0.5Zn1.9Co0.1Si2O7 Ba0.5Sr0.5Zn1.8Co0.2Si2O7 Ba0.5Sr0.5Zn1.7Co0.3Si2O7 Ba0.5Sr0.5Zn1.6Co0.4Si2O7 Ba0.5Sr0.5Zn1.5Co0.5Si2O7 Ba0.5Sr0.5Zn1.4Co0.6Si2O7 Ba0.5Sr0.5Zn1.3Co0.7Si2O7 Ba0.5Sr0.5Zn1.9Mn0.1Si2O7 Ba0.5Sr0.5Zn1.9Mn0.2Si2O7 Ba0.5Sr0.5Zn1.9Mn0.3Si2O7 Ba0.5Sr0.5Zn1.9Mn0.4Si2O7 Ba0.5Sr0.5Zn1.9Mn0.5Si2O7 Ba0.5Sr0.5Zn1.9Ni0.1Si2O7 Ba0.5Sr0.5Zn1.8Ni0.2Si2O7 Ba0.5Sr0.5Zn1.7Ni0.3Si2O7 Ba0.5Sr0.5Zn1.9Cu0.1Si2O7 Ba0.5Sr0.5Zn1.8Cu0.2Si2O7

Zn2.0 Mg0.1 Mg0.2 Mg0.3 Mg0.4 Mg0.5 Mg0.6 Mg0.7 Co0.1 Co0.2 Co0.3 Co0.4 Co0.5 Co0.6 Co0.7 Mn0.1 Mn0.2 Mn0.3 Mn0.4 Mn0.5 Ni0.1 Ni0.2 Ni0.3 Cu0.1 Cu0.2

1220−1250 1300 1300 1300 1300 1300 1270−1290 1300−1320 1200−1220 1200−1220 1200−1230 1200 1260 1200 1200−1230 1260 1150−1170 1150−1170 1120−1160 1200 1180 1180 1160 1180 1100

phase HT HT HT HT HT HT HT HT HT HT HT HT HT HT HT HT HT HT HT HT HT HT HT HT HT

+ LT

+ LT

+ LT

+ LT + LT

method is around ±0.1 × 10−6 K−1. Thermal analysis was performed with a LINSEIS DSC PT-1600, using Pt-crucibles and a rate of 10 K/ min.

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RESULTS In Table 1, the phase stability of the different compositions can be seen. High concentrations of Zn2+, i.e., low x-values, generally lead to the stabilization of phases with the crystal structure of HT-BaZn2Si2O7. The phase stability threshold, i.e., the maximum value of x, which can be reached until the LTphase is formed, strongly depends on the chosen substitute. In the case of Mg2+ and Co2+, 30% (x = 0.6) of these elements can be introduced at Zn2+-sites before the LT-phase starts to occur at room temperature. In the case of Cu2+, only 5% (x = 0.1) of Zn2+ substitution already leads to the formation of the LTphase. These solubility thresholds are at 10% (x = 0.2) and 20% (x = 0.4) in the case of Ni2+ and Mn2+, respectively. Furthermore, abbreviated sample names are given in Table 1, which will be used in the following paragraphs. XRD patterns of the solid solution series Ba0.5Sr0.5Zn2−xMgxSi2O7 are illustrated in Figure 1 together with the theoretical peak positions of the LT- as well as the HT-phase (see the lower part of Figure 1). It is obvious that the samples with x = 0.1, 0.4, and 0.6 exhibit the crystal structure of HT-BaZn2Si2O7. The samples with x = 0.3 and 0.5 are not illustrated but show nearly identical diffractograms as the samples with x = 0.4 and 0.6. The composition Ba0.5Sr0.5Zn1.3Mg0.7Si2O7 (sample Mg0.7) shows a mixture of the LT- and the HT-phase, where the latter is the minor phase. Higher Mg2+ concentrations lead to the formation of single phase materials with the crystal structure of LT-BaZn2Si2O7 B


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Inorganic Chemistry

Figure 2. Lattice parameter b in solid solutions of the form Ba0.5Sr0.5Zn2−xMxSi2O7 (M = Mg, Co, Mn, Ni, Cu) with different values of x.

The a parameter of the Mg-, Cu-, and Ni-containing samples decrease slightly with increasing substitute concentration. The value for the lattice parameter a of the Co-solid solution series remains almost constant, and in the case of Mn-containing samples, the lattice parameter increases. As is also seen in Table 2, the c parameter does not show a clear trend to higher or lower values with increasing x, especially for the samples with Co, Ni, and Cu. The Mg samples all exhibit slightly smaller lattice parameters than the Zn2.0 sample, i.e., the pure Znsample. The Mn-containing samples have higher c parameters, except for the sample Mn0.1. These changes in the lattice parameters also result in a shift of the peak positions inside the diffraction patterns. Such patterns are illustrated in Figure 3 for those samples, in which the LT-phase appears for the first time, while increasing the substitute concentration. This means, for example, in the case of the sample Mn0.5, that the samples Mn0.1, Mn0.2, Mn0.3, and Mn0.4 are single-phase materials in which only the HTphase is stable. Together with the results summarized in Table 1, it can be seen that the solubility of elements with ionic radii similar to Zn2+ decreases in the following order: Mg2+ ≈ Co2+ > Mn2+ > Ni2+ > Cu2+. The introduction of Fe2+ inside the crystal structure was tested, but it is not clear if small concentrations of this ion can be incorporated into the Ba0.5Sr0.5Zn1−xFexSi2O7 phase. Higher Fe2+-concentrations lead to the formation of iron containing secondary phases, even when synthesized in a reducing atmosphere (Argon) in order to shift the Fe2+/Fe3+ ratio to the reduced side. In order to estimate the effect of different compositions on the thermal expansion behavior, the lattice parameters were determined as a function of the temperature using hightemperature XRD (HT-XRD). This is illustrated in Figure 4 for the samples with x = 0.1 and the sample Zn2.0. The lattice parameters were fitted with polynomials of the form: y(T) = A + BT + CT2, where the temperature is given in units of °C. The respective regression parameters are summarized in Table 3. As shown in Figure 4, the effect of the Zn substitution on the temperature dependence of the lattice parameters is not large; i.e., the trend of the curves does not change much. In order to compare the thermal expansion properties, it is more common to compare the relative changes of the lattice parameters. This

Figure 1. XRD patterns of the solid solution series Ba0.5Sr0.5Zn1−xMgxSi2O7 for different values of x. In the lower part, the theoretical peak positions of phases with the crystal structures of LT- and HT-BaZn2Si2O7 are given.19,24 The latter has the composition Ba0.6Sr0.4Zn2Si2O7.

(see the sample with x = 1.0 in Figure 1). The samples with x = 1.5 and 2.0 show small concentrations of secondary phases, which could not get clearly identified. Such secondary phases were also found at high concentrations for the other solid solution samples containing Co2+, Mn2+, Ni2+, or Cu2+. However, this was not studied in detail. In the case of polyvalent substitutes, the value of x at which the phase changes occur also depends on their valence state, which should be detectable in the diffractograms because ions with a valence state differing from a value of +2 should not be incorporated in the crystal phases and, hence, should form small amounts of secondary phases. In the case of Mncontaining samples, this can be seen by the color of the resulting powder, which should be almost colorless or only slightly violet, which is a clear hint on the predominant formation of Mn2+. At higher Mn-concentrations (it was tested for the compositions Ba0.5Sr0.5ZnMnSi2O7 and Ba0.5Sr0.5Zn0.5Mn1.5Si2O7), a reduced oxygen partial pressure is needed in order to prepare samples containing only Mn2+. Otherwise, samples with brown or black color are obtained, which is a strong hint at the formation of manganese in a higher valence state. The substitution of Zn2+ also leads to changes of the lattice parameters as illustrated in Figure 2 for the lattice parameter b, which decreases with increasing x-values for all studied substitutes. This decrease in the b-parameter is different for different substitutes; the smallest influence is found for Mgcontaining samples followed by Co, Mn, Ni, and Cu. Interestingly, this is also the order in which the x-threshold decreases. The other lattice parameters scatter a lot and do not show such a distinct trend as is observed for the parameter b. The values of all lattice parameters determined at room temperature are summarized in Table 2. C


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Inorganic Chemistry

Table 2. Lattice Parameters and Unit Cell Volume Measured at Room Temperature (25−30 °C) in Ba0.5Sr0.5Zn1−xMxSi2O7 (M = Mg, Mn, Co, Ni, Cu) for Different Values of x x

lattice parameter

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

a [Å] b [Å] c [Å] V [Å3] a [Å] b [Å] c [Å] V [Å3] a [Å] b [Å] c [Å] V [Å3] a [Å] b [Å] c [Å] V [Å3] a [Å] b [Å] c [Å] V [Å3] a [Å] b [Å] c [Å] V [Å3] a [Å] b [Å] c [Å] V [Å3] a [Å] b [Å] c [Å] V [Å3]

Mg

12.96350(76) 7.69294(43) 6.57955(39) 655.53(24) 12.96414(75) 7.68720(41) 6.58121(38) 655.87(22) 12.96721(84) 7.68191(48) 6.57972(45) 655.43(26) 12.96150(71) 7.67230(40) 6.57731(36) 654.08(21) 12.96503(69) 7.66785(39) 6.58330(35) 654.47(21) 12.96548(75) 7.66510(42) 6.58303(38) 654.23(22) 12.9557(47) 7.6586(21) 6.5628(19) 651.2(12)

Co

Mn

12.96902(47) 7.68737(28) 6.57172(22) 655.00(14) 12.97045(58) 7.67777(31) 6.57526(28) 654.79(17) 12.97520(64) 7.66947(34) 6.58093(30) 654.89(18) 12.97728(77) 7.66128(43) 6.58700(37) 654.90(22) 12.97815(99) 7.65722(54) 6.59561(48) 655.45(29) 12.9815(11) 7.64566(58) 6.59352(53) 654.42(32) 12.9740(35) 7.6299(15) 6.5937(13) 652.71(87)

12.95825(25) 7.69225(15) 6.56299(11) 654.187(21) 12.96874(66) 7.68743(36) 6.57209(32) 655.69(24) 12.99849(69) 7.65866(38) 6.59489(34) 656.53(20) 13.01758(78) 7.63687(43) 6.61365(36) 657.49(22) 13.0343(13) 7.62091(70) 6.62804(59) 658.38(37) 13.0337(18) 7.61469(92) 6.62678(83) 657.69(50)

Ni

Cu

12.9572(10) 7.66338(56) 6.57805(50) 654.45(24) 12.95696(85) 7.62895(46) 6.60019(40) 652.42(24) 12.9490(46) 7.6180(20) 6.6053(18) 651.6(12)

12.96918(82) 7.65555(46) 6.58208(42) 654.17(13) 12.9769(22) 7.5802(11) 6.60152(98) 649.38(60)

3. These samples exhibit a clearly visible difference in the thermal expansion behavior compared to the sample Zn2.0. These differences are shown in Figure 7, where the relative changes of the lattice parameters and the unit cell volume of samples in which the maximum concentration of Zn was substituted are illustrated together with the sample Zn2.0. The CTE, which is correlated to the slope of the curves in Figure 7, strongly depends on the composition and is also temperature dependent; i.e., in most cases, the Δy/y0(T) curves can not be fitted using a linear regression. Hence, the CTEs depend on the temperature and the composition as illustrated in Figure 8 for the samples Zn2.0, Mg0.1, Co0.1, Mn0.1, and Ni0.1. In Figure 8, the CTE values of the respective crystallographic axes as well as the mean CTEs (αm) are illustrated. These values were calculated from 30 °C to the respective temperature, plotted at the x-axis according to technical CTE values; i.e., the CTE between 30 and 300 °C is the slope of a linear regression between the data points at 30 and 300 °C in the Δy/y0(T) plot. As also shown in Figures 5 and 7, the thermal expansion and, hence, also the CTE values depend on temperature. The introduction of Mg, Mn, Co, and Ni into the HT-BaZn2Si2O7 structure results in a decrease of the CTE in the lower temperature range in the direction of the lattice parameter b. The CTE of the a parameter is not strongly affected but also decreases slightly. At lower temperatures, the

can be calculated using the following equation for the lattice parameters y: y − y0 Δy = 1 y0 y0

where y0 is the lattice parameter at T0 (30 ± 2 °C) and y1 is the ̀ lattice parameter at T1. For the calculation of the change of the volume of the unit cell, the same equation was used, but y was replaced by the unit cell volume V. In Figure 5, the relative change of the lattice parameters is illustrated for the samples with x = 0.1. These graphs show similar trends, but the slight variations lead to a noticeable change in the thermal expansion of the complete unit cell as illustrated in the lower part of Figure 5, where it can be seen that Ni has the strongest and Mg has the smallest effect. The samples Co0.1, Mn0.1, and Ni0.1 have a smaller thermal expansion than the sample Zn2.0. The sample Mg0.1 exhibits a thermal expansion behavior similar to that of Zn2.0. Compositions containing Co, Mn, and Ni also exhibit negative thermal expansion. However, this is only valid in the lower temperature range up to around 400 or 500 °C. At higher temperatures, the relative change of the volume of the unit cell is equal within the measuring accuracy. In Figure 6, the lattice parameters of the samples Mg0.6, Co0.6, Mn0.4, Ni0.2, and Cu0.1 are shown between 30 and 1000 °C. The regression parameters can also be found in Table D


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Inorganic Chemistry

In Figure 9, the CTE values of the crystallographic axes are shown together with the mean CTEs. In contrast to Figure 8, the compositions are those of the maximum possible substitute concentration, where the HT-phase still occurs. These are the samples Zn2.0, Mg0.6, Mn0.4, Co0.6, Ni0.2, and Cu0.1. In the lower temperature range, the substitution leads to a notable decrease of the b-parameter. The expansion of the b-parameter near room temperature is even more negative than for the lower substitute concentrations displayed in Figure 8. The respective CTEs from 30 to 300 °C as well as 30 to 800 °C are given in Table 4 together with the mean CTEs and a parameter A, defined as the difference between the highest and the lowest CTE of the respective composition. It is a measure of the anisotropy of the thermal expansion. The mean values of the CTEs between room temperature and 300 °C are also illustrated in Figure 10. The mean CTE between 30 and 300 °C is the highest for the Ba0.5Sr0.5Zn2Si2O7 and decreases for all compositions in which Zn2+ is partially substituted, as shown in Figures 8 and 9 as well as in Table 4. The sample Co0.6 has the smallest mean CTE of all studied compositions (−2.9 × 10−6 K−1). However, a low CTE up to 300 °C does not automatically result in low CTEs up to 800 °C as it can be seen for the Ni0.2 sample with a low CTE of −1.9 × 10−6 K−1 (30−300 °C) and a high CTE of 2.1 × 10−6 K−1 (30−800 °C). Also, the anisotropy is affected by the composition and is larger in the lower temperature range for all samples in which Zn was partially replaced. Ba0.5Sr0.5Zn1.9M0.1Si2O7 samples have an anisotropy of between 58.9 and 64.9 × 10−6 K−1. Except for the samples containing Ni2+, the anisotropy increases with decreasing Zn2+-concentration and is generally highest at lower temperatures.

Figure 3. XRD patterns of Ba0.5Sr0.5Zn2−xMxSi2O7 solid solutions in which both LT- and HT-BaZn2Si2O7 are stabilized at room temperature.

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DISCUSSION If 50% of Ba2+ in BaZn2Si2O7 is substituted by Sr2+, the structure of the HT-phase of BaZn2Si2O7 is stable at room temperature, which leads to a negative CTE in the direction of the lattice parameter b and in some cases also to an overall contraction of the volume of the unit cell. A certain concentration of Zn2+ in the Ba0.5Sr0.5Zn2Si2O7 lattice can be substituted by Mg2+, Co2+, Mn2+, Ni2+, or Cu2+, while the HTphase of BaZn2Si2O7 with its negative thermal expansion properties is still stable at room temperature. If a certain threshold concentration of the above-mentioned substitutes is exceeded, the structure changes to that of LT-BaZn2Si2O7, which shows a highly positive CTE. The change in crystal structure is shown in Figure 1 for the incorporation of Mg2+ into the crystal phase at a value o f x = 0.7 (Ba0.5Sr0.5Zn1.3Mg0.7Si2O7). The diffraction peaks match approximately the calculated ones but exhibit small shifting (versus the HT-BaZn2Si2O7) which is a result of the formation of solid solutions affecting the lattice constants and hence the peak positions in this system.19 At this composition, only small concentrations of the phase with HT-BaZn2Si2O7 structure are found, but this strongly depends on the sample preparation, i.e., the applied synthesis temperatures and the homogeneity of the as prepared powder, which both affect the distribution of Ba, Sr, Zn, and the substitutes. Both the Ba/Sr ratio and the Zn/ substitute ratio strongly affect the phase stability. However, in this study, the Ba/Sr ratio was kept constant in order to evaluate the effect of the Zn substitution. In the solid solution Ba0.5Sr0.5Zn2−xMxSi2O7, a step width of Δx = 0.1 was chosen. In order to evaluate a more precise composition at which both the

Figure 4. Lattice parameters of Ba0.5Sr0.5Zn2Si2O7 and solid solutions of the form Ba0.5Sr0.5Zn1.9M0.1Si2O7 with M = Mg, Co, Mn, and Ni determined using HT-XRD.

thermal expansion of the c parameter is larger for nearly all compositions than that for the sample Zn2.0. E


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Inorganic Chemistry

Table 3. Regression Parameters Describing the Temperature Dependence of the Lattice Parameters of Different Samples Using a Polynomial of the Form: y(T) = A + BT + CT2 regression parameters A [Å] sample Zn2.0

Mg0.1

Mg0.6

Co0.1

Co0.6

Mn0.1

Mn0.4

Ni0.1

Ni0.2

Cu0.1

lattice parameter a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c

value 12.964 7.705 6.562 12.959 7.706 6.569 12.965 7.673 6.577 12.968 7.694 6.569 12.984 7.652 6.591 12.978 7.698 6.570 13.029 7.626 6.625 12.966 7.676 6.581 12.958 7.639 6.600 12.975 7.668 6.580

C [Å/°C2]

B [Å/°C] std. error

1.89 1.45 9.60 1.72 2.63 1.55 1.15 1.92 9.59 2.42 1.33 7.53 1.05 2.58 1.64 1.02 1.53 1.62 1.51 2.77 1.26 8.73 1.80 1.82 1.53 3.83 1.17 7.82 2.54 1.40

× × × × × × × × × × × × × × × × × × × × × × × × × × × × × ×

−3

10 10−3 10−4 10−3 10−3 10−3 10−3 10−3 10−4 10−3 10−3 10−4 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−4 10−3 10−3 10−3 10−3 10−3 10−4 10−3 10−3

value 1.12 −3.09 1.59 1.57 −3.51 1.84 1.03 −3.81 1.95 1.25 −3.25 1.64 9.07 −3.69 1.70 9.98 −3.38 1.62 8.88 −3.38 1.78 1.01 −3.40 1.58 1.11 −3.33 1.57 9.02 −3.48 1.69

LT- and the HT-phase are stable, a smaller Δx might be chosen. The crystal structures reported here are similar or equal to HT- and LT-BaZn2Si2O7. The crystal structures of both phases are reported in the literature, but in the case of HTBaZn2Si2O7, different space groups are reported.16,19 The crystal structures were not refined, but all diffraction peaks of the HT- and LT-phase were also found for the investigated compositions; additionally, a refinement of the diffractograms from which the lattice parameters were determined leads to a reliable fit. Hence, it can be concluded that the samples reported here have a crystal structure of HT- or LTBaZn2Si2O7. As mentioned above, in the case of a complete or almost complete substitution of Zn2+, also other phases appear. The lattice parameters a and c of the solid solution phases with the crystal structure of HT-BaZn2Si2O7, in which Zn2+ is substituted, do not show a clear trend. These parameters are affected not as strongly by compositional changes than the b parameter as illustrated in Figure 2. A slight compositional inhomogeneity might lead to these local variations within the solid solutions. In the direction of the crystallographic b-axis, the lattice parameter depends much more on the composition than in the other crystallographic directions. For all compositions illustrated in Figure 2, the error in the lattice constant b is smaller than the symbol size and also in the case of

× × × × × × × × × × × × × × × × × × × × × × × × × × × × × ×

std. error −4

10 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−5 10−4 10−4 10−5 10−4 10−4 10−5 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−5 10−4 10−4

4.39 8.76 5.11 9.76 1.48 7.70 2.33 1.01 4.68 1.50 8.15 4.05 2.30 1.55 9.11 2.20 8.30 7.67 2.85 1.43 6.65 1.93 9.86 9.02 2.98 2.00 5.80 1.77 1.52 7.54

× × × × × × × × × × × × × × × × × × × × × × × × × × × × × ×

value

std. error

−6

10 10−6 10−6 10−6 10−5 10−6 10−6 10−5 10−6 10−5 10−6 10−6 10−6 10−5 10−6 10−6 10−6 10−6 10−6 10−5 10−6 10−6 10−6 10−6 10−6 10−5 10−6 10−6 10−5 10−6

1.36 −2.31 −6.89 1.82 −4.00

× × × × ×

10−7 10−8 10−8 10−7 10−8

9.64 5.23 1.06 1.58 7.62

× × × × ×

10−9 10−9 10−8 10−8 10−9

2.07 −5.27 −3.11 1.61 −3.09

× × × × ×

10−7 10−8 10−8 10−7 10−8

1.01 4.65 1.69 9.08 4.25

× × × × ×

10−8 10−9 10−8 10−9 10−9

1.95 × 10−7 −3.04 × 10−8

1.63 × 10−8 9.81 × 10−9

1.71 × 10−7 −2.85 × 10−8

8.62 × 10−9 7.50 × 10−9

2.08 × 10−7 −5.60 × 10−8

1.47 × 10−8 6.90 × 10−9

1.87 × 10−7 −3.25 × 10−8

1.02 × 10−8 9.00 × 10−9

1.98 × 10−7 −3.49 × 10−8

1.98 × 10−8 5.83 × 10−9

1.92 × 10−7 −3.60 × 10−8

1.62 × 10−8 7.88 × 10−9

the compositions with the highest x-value, where an additional phase appeared. As mentioned above, the phase stability, i.e., the maximum value of x, which still enables one to prepare a phase with the structure of HT-BaZn2Si2O7, i.e., before a phase with the structure of LT-BaZn2Si2O7 starts to form, correlates with the decrease in the lattice parameter b. Hence, a more pronounced decrease in the lattice parameter b is correlated with a lower solubility of the respective bivalent ion substituting Zn2+. In the direction of the lattice parameter b, the negative thermal expansion can be found, and in this direction, also the change in length occurring during the transformation from the LT-phase to the HT-phase is the highest.14,19 It should be noted that, according to slightly different refinement results for crystals with a structure of HT-BaZn2Si2O7, the crystallographic direction of negative thermal expansion is defined differently. In ref 16, this was defined as the a parameter while the same axis was denoted as the b parameter in ref 19. The latter definition is also used in this study. In this direction, also the change in length during the phase transition is the highest. It is known from the literature that, in the case of BaZn2−xMxSi2O7 (M = Mg, Co, Ni) solid solutions, the partial or complete substitution of Zn2+ for Mg2+, Co2+, and Ni2+ leads to smaller volume effects due to the phase transition from the low to the high temperature phase.12,14,15 In the case of Ba0.5Sr0.5Zn2−xMxSi2O7 (M = Mg, Co, Mn, Ni, Cu) solid solutions, the substitution of F


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Inorganic Chemistry

Figure 5. Relative change of the lattice parameters and the volume of the unit cell of Ba0.5Sr0.5Zn2Si2O7 and solid solutions of the form Ba0.5Sr0.5Zn1.9M0.1Si2O7 (M = Mg, Co, Mn, and Ni) as a function of the temperature.

Figure 7. Relative change of the lattice parameters and the volume of the unit cell of Ba0.5Sr0.5Zn2Si2O7 and solid solutions of the form Ba0.5Sr0.5Zn2−xMxSi2O7 (M = Mg, Co, Mn, and Ni) as a function of the temperature. In these samples, the maximum concentration of the elements M is introduced. Higher concentrations lead to the stabilization of crystal phases with the structure of LT-BaZn2Si2O7.

Figure 8. Technical CTEs calculated from 30 °C to the temperature plotted at the x axis. On the left, pure Ba0.5Sr0.5Zn2Si2O7 is shown. The other diagrams show the CTEs of Ba0.5Sr0.5Zn1.9M0.1Si2O7 solid solutions with M = Mg, Mn, Co, and Ni; i.e., samples in which 5% of Zn2+ were replaced by the as mentioned bivalent metal ions. αa, αb, αc, and αm are the CTEs in the a, b, and c direction as well as the mean CTEs, respectively.

Figure 6. Lattice parameters of solid solutions of the form Ba0.5Sr0.5Zn2−xMxSi2O7 with M = Mg, Co, Mn, Ni, and Cu determined with HT-XRD. The nonsubstituted sample Zn2.0 as well as the samples containing the maximum concentration of the elements M are illustrated. Higher concentrations lead to the stabilization of crystal phases with the structure of LT-BaZn2Si2O7.

Zn leads to the effect that the values of the lattice parameters of the high temperature phase approach the lattice parameters of G


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Inorganic Chemistry

Concerning the LT-BaZn2Si2O7 crystals, the substitution of Zn2+ leads to an enlargement of the phase stability range; i.e., the phase transition temperatures can be shifted to higher temperatures with increasing substitute concentration. The phase transition is the lowest for the pure Zn-containing compound and the highest for the Ni- and Cu-containing compositions. The phase transition temperatures are given in Table 5 for the BaZn2−xMxSi2O7 solid solutions with x = 0.1 and 0.2. The introduction of Cu2+ leads to the most pronounced increase of the phase transition temperature, while it is also the substitute with the lowest solubility in the HT-BaZn2Si2O7 structure. Mg2+ and Co2+ have a much lower effect on the phase transition temperature of BaZn2Si2O7; i.e., these ions do not stabilize the LT-phase as much and can hence be incorporated in a higher concentration in the HTBaZn2Si2O7 structure before the LT-phase starts to form. This explanation does not fit with the effect of Mn2+ on the phase transition temperature (see Table 5), which is similar to that of Co2+ and Mg2+. A reason for this behavior might be the polyvalency of Mn; i.e., small amounts Mn with higher valence states might have formed so that the phase transition temperatures in Table 5 might be underestimated. The thermal expansion behavior of the samples with high Zn-concentrations (Mg0.1, Co0.1, Mn0.1, and Ni0.1) illustrated in Figure 4 shows only a slight effect of the composition on the thermal expansion in the different crystallographic directions. A decrease of the thermal expansion of the overall unit cell volume can be seen in the lower part of Figure 5. This effect is the highest for the elements, which have the lowest solubility in crystals with the HT-BaZn2Si2O7 structure. This decrease of the thermal expansion becomes more pronounced with higher substitute concentration as illustrated in Figures 6 and 7. The mean CTEs also decrease with increasing substitute concentration as shown in Figures 8 and 9. A decrease in the mean value correlates with an increase in the anisotropy of thermal expansion. A lower mean CTE is advantageous for the development of materials with zero thermal expansion, but the very high anisotropy might lead to the formation of microcracks during thermal cycling, making the development of a dense and crack-free material from the as described solid solution phases a challenging task. However, the usage of small cracks in order to control the thermal expansion might also be possible but leads to comparatively low strength.23 The usage of such phases, for example, in composite materials with zero thermal expansion,

Figure 9. Technical CTEs of solid solutions with the crystal structure of HT-BaZn2Si2O7 calculated from 30 °C to the temperature plotted at the x axis. On the left, pure Ba0.5Sr0.5Zn2Si2O7 is illustrated. The other diagrams show the CTEs of Ba0.5Sr0.5Zn2−xMxSi2O7 solid solutions with M = Mg, Mn, Co, Ni, and Cu. Except for the left sample in which only ZnO is introduced, the values of x are as high as possible; i.e., as much ZnO as possible is replaced by the other oxides. αa, αb, αc, and αm are the CTEs in the a, b, and c direction as well as the mean CTEs, respectively.

the low temperature phase. If a certain degree of substitution is reached, the LT-phase forms. According to ref 22, the ionic radii in 4-fold coordination decrease in the following order: Mn2+ > Zn2+ > Co2+ > Mg2+ = Cu2+ > Ni2+. The value of Fe2+, which can not (or only in small concentrations) be incorporated into the structure, is between those of Mn2+ and Zn2+. In the substitutes studied in this paper, the decrease in the lattice parameter increases in the order: Mg, Co, Mn, Ni, and Cu. Hence, the order in which the b-parameter is changed (see Figure 2) does not correlate with the change in the ionic radii. In analogy, the maximum concentration of Znsubstitutions in solid solutions with the structure of HTBaZn2Si2O7 can not be correlated to the ionic radii. However, the volume of the unit cell (see Table 2) can roughly be correlated to the ionic radii; i.e., the Mn-containing samples exhibit the highest unit cell volume and the Ni2+-containing samples have the lowest. The solubility of the different Zn-substitutes in Ba0.5Sr0.5Zn2−xMxSi2O7 solid solutions depends on the chosen substitute as illustrated in Figure 3. The solubility of Co2+ and Mg2+ is the highest and the solubility of Cu2+ is the lowest, which correlates with the decrease of the lattice parameter b.

Table 4. Technical CTEs in Different Crystallographic Directions a, b and c and the Mean CTE αma 30 to 300 °C [10−6 K−1]

30 to 800 °C [10−6 K−1]

sample

αa

αb

αc

αm

A

αa

αb

αc

αm

A

Zn2.0 Mg0.1 Mg0.6 Co0.1 Co0.6 Mn0.1 Mn0.4 Ni0.1 Ni0.2 Cu0.1

10.5 9.8 9.3 10.4 7.3 8.0 8.2 8.0 10.0 7.6

−32.4 −34.5 −42.0 −34.5 −42.7 −35.1 −39.4 −38.1 −39.1 −39.3

23.9 26.1 28.4 24.4 26.7 26.0 24.9 25.3 23.4 25.6

0.7 0.5 −1.4 0.1 −2.9 −0.3 −2.1 −1.6 −1.9 −2.0

56.3 60.6 70.4 58.9 69.4 61.1 64.3 63.4 62.5 64.9

8.4 7.7 7.9 7.3 7.1 7.5 6.9 8.0 8.7 6.8

−24.8 −25 −26.5 −24.2 −26.5 −24.5 −21.6 −23.1 −21.7 −23.6

21.3 22.4 23.1 20.9 21.9 21.2 19.5 19.8 19.4 20.8

1.6 1.7 1.5 1.3 0.8 1.4 1.6 1.5 2.1 1.3

46.1 47.4 49.6 45.1 48.4 45.7 41.1 42.9 41.1 44.4

The values are given for two temperatures ranges, which are 30 to 300 °C as well as 30 to 800 °C. The value A describes the anisotropy of the thermal expansion, which is the difference between the highest and the lowest CTE.

a

H


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Inorganic Chemistry

Figure 10. Mean CTEs measured in a temperature range from 30 to 300 °C for different compositions.

■

Table 5. Phase Transition Temperatures in BaZn2−xMxSi2O7 Solid Solutions (x = 0.1 and 0.2) Determined with Dilatometry

*E-mail: [email protected]. Phone: 0049 (0) 3641 948525. Fax: 0049 (0) 3641−94 85 02.

phase transition temperature [°C] M

x = 0.1

x = 0.2

ref

Mg Co Mn Ni Cu

334 332 328 447 482

379 375 355 575 610

14 15a

AUTHOR INFORMATION

Corresponding Author

Notes

The authors declare no competing financial interest.

■

REFERENCES

(1) Takenaka, K. Sci. Technol. Adv. Mater. 2012, 13, 013001. (2) Barrera, G. D.; Bruno, J. A. O.; Barron, T. H. K.; Allan, N. L. J. Phys.: Condens. Matter 2005, 17, R217−R252. (3) Salvador, J. R.; Guo, F.; Hogan, T.; Kanatzidis, M. G. Nature 2003, 425, 702−705. (4) Guedes, M.; Ferro, A. C.; Ferreira, J. M. F. J. Eur. Ceram. Soc. 2001, 21, 1187−1194. (5) Grima, J. N.; Zammit, V.; Gatt, R. Xjenza 2006, 11, 17−29. (6) Himei, Y.; Nagakane, T.; Fukumi, K.; Kitamura, N.; Nishii, J.; Sakamoto, A.; Hirao, K. J. Non-Cryst. Solids 2008, 354, 3113−3119. (7) Soares, V. O.; Peitl, O.; Zanotto, E. D. J. Am. Ceram. Soc. 2013, 96, 1143−1149. (8) Hu, A.-M.; Liang, K.-M.; Wang, G.; Zhou, F.; Peng, F. J. Therm. Anal. Calorim. 2004, 78, 991−997. (9) Arnault, L.; Gerland, M.; Rivière, A. J. Mater. Sci. 2000, 35, 2331− 2345. (10) Guo, X.; Yang, H.; Han, C.; Song, F. Thermochim. Acta 2006, 444, 201−205. (11) Thieme, C.; Rüssel, C. J. Power Sources 2014, 258, 182−188. (12) Thieme, C.; Rüssel, C. J. Mater. Sci. 2015, 50, 3416−3424. (13) Thieme, C.; Rüssel, C. Thermochim. Acta 2015, 612, 49−54. (14) Kerstan, M.; Müller, M.; Rüssel, C. J. Solid State Chem. 2012, 188, 84−91. (15) Kerstan, M.; Thieme, C.; Grosch, M.; Müller, M.; Rüssel, C. J. Solid State Chem. 2013, 207, 55−60. (16) Lin, J. H.; Lu, G. X.; Du, J.; Su, M. Z.; Loong, C.-K.; Richardson, J. W., Jr J. Phys. Chem. Solids 1999, 60, 975−983. (17) Adams, R. D.; Layland, R.; Payen, C.; Datta, T. Inorg. Chem. 1996, 35, 3492−3497. (18) Park, C.-H.; Choi, Y.-N. J. Solid State Chem. 2009, 182, 1884− 1888. (19) Thieme, C.; Görls, H.; Rüssel, C. Sci. Rep. 2015, 5, 18040. (20) Thieme, C.; Rüssel, C. Ceram. Int. 2015, 41, 13310−13319. (21) Kondo, S.; Tateishi, K.; Ishizawa, N. Jpn. J. Appl. Phys. 2008, 47, 616−619. (22) Shannon, R. D. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, 32, 751−767. (23) Parker, F. J. J. Am. Ceram. Soc. 1990, 73, 929−932. (24) Segnit, E. R.; Holland, A. E. Aust. J. Chem. 1970, 23, 1077−1085.

12a

a Calculated from a second order polynomial fit of the onsets of the phase transitions.

however, should be much easier. Therefore, a ceramic powder with the composition Ba0.5Sr0.5Zn1.4Co0.6Si2O7 should be the most suitable composition because of the lowest CTE of −2.9 × 10−6 K−1 in the range from 30 to 300 °C.

■

CONCLUSION The thermal expansions and phase stabilities of Ba0.5Sr0.5Zn2−xMxSi2O7 solid solutions were studied using XRD techniques. The solubility of the elements M decreases in the following order: Mg2+ ≈ Co2+ > Mn2+ > Ni2+ > Cu2+. The incorporation does not solely depend on the ionic radii because Fe2+ can not (or only in small concentrations) be introduced into the crystal structure. All substitutes decrease the mean CTEs in the lower temperature range up to around 500 °C. Above this temperature, all samples exhibit a similar and almost temperature independent thermal expansion behavior. The lowest CTEs were reached for the samples with the lowest Zn-concentrations; i.e., the most negative CTEs can be obtained just below the phase stability threshold at which phases with the structure of LT-BaZn2Si2O7 and high thermal expansion start forming. The negative overall thermal expansion correlates with a very large anisotropy of thermal expansion, which increases with decreasing Zn-concentration. The CTEs of the lattice parameter b are in the range between −42.7 and −32.4 × 10−6 K−1 (30−300 °C), and those in the direction c are in between 23.4 and 28.4 × 10−6 K−1 measured in the same temperature range. The low mean CTEs make these phases suitable for zero thermal expansion materials or as filler components to achieve zero thermal expansion in composite materials. I


New Family of Materials with Negative Coefficients of Thermal

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