The XRD patterns of (a) Mg 5 Nb 4 O 15 , (b) Mg 5 Ta 4 O 15 , (c) 

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... could not be noticed. The compounds crystallize with the orthorhombic structure within the space group D 17 2 h À C mcm with Z = 4. The structure consists of double chains of (Mg, B)O 6 units, sharing edges of the bc plane, interconnected through common oxygen along the a -axis to give a three- dimensional array [15]. The microwave dielectric properties of one of the compounds (Ba 5 Nb 4 O 15 ) in the A 5 B 4 O 15 are reported earlier [16,17]. In the present report, we make a detailed study of the preparation, characterization and microwave dielectric properties of title compounds A 5 B 4 O 15 (A=Ba, Sr, Mg, Ca, Zn; B=Nb, Ta). The ceramics were prepared through the conven- tional solid-state ceramic route. The high purity carbonates or oxides i.e., BaCO 3 (99.5% Aldrich Chemicals), SrCO 3 (99.9% Aldrich), CaCO 3 (>99.5% Aldrich Chemicals), MgO (99+%, CDH India), ZnO (99.99%, Aldrich Chemicals), Nb 2 O 5 (99.9%, NFC, Hyderabad, India) and Ta 2 O 5 (99.9%, NFC) were used. The MgO is calcined at 1000 j C for 3 h to remove hydroxides or carbonates [26]. Mg 5 Nb 4 O 15 and Mg 5 Ta 4 O 15 were prepared using both calcined MgO and un-calcined MgO. The oxide or carbonate powders were weighed as per the molar ratios to get a gross amount of about 20 g, mixed thoroughly in agate mortar using distilled water or acetone as the wetting medium for a duration of 1 h, dried and again mixed for 1 h. The reaction mixtures of the niobates were calcined at 1050 –1275 j C and tanta- lates at 1200– 1400 j C for a duration ranging from 4 to 8 h. The calcined mixture is ground well for 1 h, 3 wt.% PVA is added, dried and again ground. The fine powder is uni-axially pressed at a pressure of 150 MPa using a tungsten carbide die of 11-mm diameter. The dimensions of the ceramic compacts are controlled such that the sintered body has aspect ratio (D/L) of 1 to 1.3 or 2 to 2.3 to obtain maximum mode separation during measurements. Stearic acid dissolved in isopropyl alcohol is used as lubricant while pressing. This can reduce the friction between powder and die wall. The samples are heated at the rate 270 j C/h up to 800 j C for burning out PVA and then fast heating rate of 600 j C/h is applied up to the sintering temperature. The pellets were sintered in the temperature range 1175– 1625 j C for 2 to 4 h. The sintering temperatures are optimised to get maximum density for constant duration. The sintered densities of the samples were measured using Archi- medes method. The pellets were polished well and shaped to avoid any surface irregularities. The X-ray diffraction spectra of the sintered samples were recorded after grinding them into fine powder. The spectra were recorded using a Philips X-ray diffractometer with Cu K a radiation with Ni filter. The surface of the sintered specimens were analysed through scanning electron micrograph (SEM) using a JEOL Scanning Electron Microscope. For the SEM studies, the finely polished samples were thermally etched at 25– 50 j C less than their respective sintering temperature for 30 min. The surface is gold coated before recording SEM. The microwave dielectric properties of the compounds are measured using an HP 8510 C Network Analyser. The dielectric constant and temperature coefficient of resonant frequency are measured using the Hakki– Coleman method [18]. The quality factor is measured using a transmission mode cavity [19]. Table 1 gives the calcination temperature, sintering temperature, density and percentage density of all the ceramics. The ceramics could be sintered into dense bodies. Most of the compounds have sintered densities more than 93% of their theoretical densities. The pure Mg 5 Nb 4 O 15 and Mg 5 Ta 4 O 15 phase are obtained by calcining at 1300 j C for 8 h and at 1400 j C for 8 h, respectively, using calcined MgO. The Ba 5 Ta 4 O 15 , Ba 5 Nb 4 O 15 , Sr 5 Nb 4 O 15 and Sr 5 Ta 4 O 15 are hexagonal structured in agreement with the earlier reports. When uncalcined MgO was used, the sintered product contained MgNb 2 O 6 and MgTa 2 O 6 as the secondary phases. Single phase Mg 5 Ta 4 O 15 could not densify more than 91% with- out additives. CeO 2 , Nd 2 O 3 , Sm 2 O 3 , MnO 2 and Bi 2 O 3 (1 wt.%) are tried as sintering aids to Mg 5 Nb 4 O 15 and Mg 5 Ta 4 O 15 , both prepared from MgO heat treated to 1000 j C for 3 h. Table 2 shows the densities of the compounds with 1 wt.% of the sintering aids. The addition of sintering aids did not increase the sintered density in the case of Mg 5 Nb 4 O 15 . However, 1 wt.% Bi 2 O 3 added as sintering aid has improved the density to 96% in the case of Mg 5 Ta 4 O 15 . All the ceramics gave single phase except the compounds of calcium and zinc. Attempts to prepare Zn 5 Nb 4 O 15 were not successful, but resulted in multiphase. The multiphase ceramics were a mixture of ZnNb 2 O 6 and Zn 3 Nb 2 O 8 . The calcium-based ceramics Ca Nb O and Ca Ta O also did not form. The different phases present in the resultant ceramics could not be identified. Fig. 1 shows the XRD patterns of Mg 5 Nb 4 O 15 , Mg 5 Ta 4 O 15 , 5CaO – 2Nb 2 O 5 , 5CaO – 2Ta 2 O 5 and 5ZnO – 2Nb 2 O 5 (ZnNb 2 O 6 +Zn 3 Nb 2 O 8 ). Fig. 2 shows the XRD patterns of Ba 5 Ta 4 O 15 , Sr 5 Nb 4 O 15 , Ba 5 Nb 4 O 15 and Sr 5 Ta 4 O 15 . Fig. 3 shows the SEM pictures of Mg 5 Nb 4 O 15 , Mg Ta O and 5CaO – 2Nb O . The presence of porosity of Mg 5 Nb 4 O 15 and Mg 5 Ta 4 O 15 is evident from the SEM. The grains are relatively large in size of about 20 A m. Fig. 4 shows the SEM picture of 5ZnO – 2Nb 2 O 5 (ZnNb 2 O 6 +Zn 3 Nb 2 O 8 ). The grains are very large up to 40 A m in size. The 5ZnO – 2Nb 2 O 5 ceramics is dense. A possible liquid phase sintering might have taken place, which is the cause for bigger grains and lower porosity. The presence of two types of grains is evident in Fig. 4. The ceramics showed good resonance at microwave frequencies. The microwave dielectric properties of the ceramics were summarised in Table 3. The ceramics have e r in the range 11– 51, Q Â f in the range 2400– 88,000 GHz and s f in the range À 73 – +232 ppm/ j C. The Ba 5 Ta 4 O 15 has a lower e r = 28 than that of analogous Ba 5 Nb 4 O 15 , which has e r = 39. However, this is in contrary to the expectation that the tantalum analogue should have higher dielectric constant than the niobium compound due to the larger ionic polarisability [20,21] of tantalum com- pared to niobium which provided both the ceramics crystallise in the same symmetry group. Spectro- scopic studies by Massa et al. [22,23] shows that the lattice of Ba 5 Ta 4 O 15 is stable whereas that of Ba 5 Nb 4 O 15 is going to collapse to a lower symmetry state and hence there may be increased lattice anhar- monicity in the compound. This may be reason for the higher dielectric constant of Ba 5 Nb 4 O 15 . It is interesting to note that the s f of the Ba 5 Ta 4 O 15 (+12 ppm/ j C) is considerably lower than that of Ba 5 Nb 4 O 15 (+78 ppm/ j C). Orthorhombic structured Mg 5 Nb 4 O 15 and Mg 5 Ta 4 O 15 showed a comparatively lower dielectric constant of 11 than the hexagonal phases that we have discussed and having the general formula A 5 B 4 O 15 . The lower dielectric constant may be due to the lower ionic polarisability of Mg ions and their different structures. The phase pure Mg 5 Nb 4 O 15 and Mg 5 Ta 4 O 15 ceramics have Q Â f up to 37,400 GHz and s f of À 54 ppm/ j C each. In the case where uncalcined MgO is used, the Mg deficiency in the above ceramics leads to MgNb 2 O 6 and MgTa 2 O 6 as the secondary phases. Presence of MgTa 2 O 6 whose e r = 30.3, Q Â f = 59,600 GHz and s f = +30 ppm/ j C [24] increases dielectric constant of Mg 5 Ta 4 O 15 (prepared using uncalcined MgO) into 17 where as its s f decreases to À 15 ppm/ j C. In a similar way, deficiency of Mg in Mg 5 Nb 4 O 15 (non-stoichiometry) leads to the formation of MgNb 2 O 6 as the secondary phase which has e r = 21.4, Q Â f = 93,800 GHz and s f = À 70 ppm/ j C [24]. The presence of the MgNb 2 O 6 secondary phase increases the dielectric constant for 11 to 14 and s f from À 54 to À 58 ppm/ j C, but decreases the quality factor than the pure compound. The phase pure Mg 5 Ta 4 O 15 could not densify more than 91%. Hence, we have added 1 wt.% Nd 2 O 3 , Sm 2 O 3 , Bi 2 O 3 , CeO 2 , and MnO 2 into powders of Mg 5 Nb 4 O 15 and Mg 5 Ta 4 O 15 and then studied the densification and microwave dielectric properties. The results are summarised in Table 2. In the case of Mg 5 Nb 4 O 15 , the addition of Nd 2 O 3 , Sm 2 O 3 , Bi 2 O 3 , CeO 2 , and MnO 2 all decreased the density but slightly increased the dielectric constant whereas the Q Â f deteriorated. In the case of Mg 5 Ta 4 O 15 , 1 wt.% of Nd 2 O 3 , Bi 2 O 3 and MnO 2 increased the density but CeO 2 and Sm 2 O 3 decreased the density. The presence of additives increased the dielectric constant. Addition of 1 wt.% CeO 2 has increased the Q Â f of 18,600 GHz, but other additives decreased the quality factor. Though the addition of 1 wt.% of Bi 2 O 3 has increased the density and dielectric constant, it reduced the Q factor. The 5ZnO – 2Nb 2 O 5 composition does not give single-phase compounds analogous to A 5 B 4 O 15 (Zn 5 Nb 4 O 15 ). Instead they give a mixture of ZnNb 2 O 6 and Zn 3 Nb 2 O 8 . The ZnNb 2 O 6 is reported to have e r = 25, Q Â f = 83,700 GHz and s f = À 56 ppm/ j C [24]. The Zn 3 Nb 2 O 8 has e r about 22, Q Â f = 83,300 GHz and s f = À 71 ppm/ j C [25]. The 5ZnO –2Nb 2 O 5 showed e r = 21, Q Â f = 88,000 GHz and s f = À 73 ppm/ j C. Similarly, single phases analogous to A 5 B 4 O 15 (i.e., Ca 5 Nb 4 O 15 ) could not be obtained for 5CaO – 2Nb 2 O 5 and 5CaO –2Ta 2 O 5 . However, the ceramics show good microwave dielectric properties and are given in Table 3. The 5CaO – 2Nb 2 O 5 has e r = 32, Q Â f = 6500 GHz and s f = À 37 ppm/ j C, whereas 5CaO – 2Ta 2 O 5 has e r = 41, Q Â f = 5900 GHz and s = +140 ppm/ j C. The A 5 B 4 O 15 (A = Ba, Sr, Mg, Ca, Zn; B = Nb, Ta) ceramics are prepared through the ...