J. P. COUGHLIN AND RAYMOND L. O m
530 [CONTRIBUTION FROM
THE
VOl. 75
MINERALSTHERMODYNAMICS BRANCH,REGION111, BUREAUOR MINES, UNITEDSTATES DEPARTMENT O F THE INTERIOR]
High Temperature Heat Contents of Meta- and Orthotitanates of Barium and Strontium BY J. P. COUGHLIN AND R. L. O m RECEIVED AUGUST11, 1952 High temperature heat content measurements of crystalline barium metatitanate, strontium metatitanate, barium orthotitanate and strontium orthotitanate were conducted over the temperature range from 298 to above 1800'K. The results are regular, except those for barium metatitanate, which has a Curie-point near 400'K. involving a small excess heat absorption. Heat content and entropy increment data above 298.16'K. are given in tabular form and heat content equations are derived, for use in the two common methods of making thermodynamic calculations.
Recent papers of Todd and Lorenson' presented low temperature heat capacity data and entropies a t 298.16"K. for the pure, crystalline meta- and orthotitanates of barium and strontium and for two solid solutions. The present paper reports high temperature heat contents for the same pure titanates from 298.16"K. to temperatures in the range 1820-1832'K. Materials and Methods The titanates used in this work were portions of the samples employed by Todd and Lorenson,' and the reader is referred to their papers for details of the preparations and analyses. The heat content measurements were made with apparatus described* previously. The substances were enclosed in gas-tight, sealed platinum-rhodium capsules during the measurements. The heat contents of the empty capsules were determined in separate experiments. Frequent calibrations of the furnace thermocouple were made a t the melting point of gold, and occasional calibrations at the melting point of palladium.
Results The measured heat content data are given in Table I, expressed in defined calories (1 cal. = 4.1833 int. joules) per mole. The heat content of barium metatitanate is higher than that of strontium metatitanate by amounts ranging from 3.2 to 6.5%, depending upon the temperature. Likewise, the heat content of barium orthotitanate is higher than that of strontium orthotitanate by 4.7 to 6.0%. Barium metatitanate has a Curie-point just below 400'K., which has been reported by several previous workers. Blattner and Merz3 gave 393°K. as the temperature, and 47 cal./mole as the excess heat absorption in the heat capacity "hump." Harwood, Popper and Rushman4gave 398"K., and 0.58 cal./deg./mole as the maximum excess heat capacity. Kanzig and Meier6 gave 392°K. an'd Sawada and Shiranee gave 378-388°K. In the region of this Curie-point the structure changes from pseudocubic to cubic, and the dielectric constant and heat capacity pass through maxima. The heat capacity maximum for barium metatitanate is involved in the present measurements as only a second-order effect, because the excess heat absorption is only a small fraction (ca. 2%) of the heat required to warm the substance from 298.16 to (1) S. S. Todd and R. E. Lorenson, THISJOURNAL, 74, 2043, 3764 (1952). (2) K. K. Kelley, B. F. Naylor and C. H. Shomate, U. S . Bur. Mines Tech. Paper 686 (1946). (3) H. Blattner and W. M e n , Helv. Phys. Acta, 2 1 , 210 (1948). (4) M.G.Harwood, P. Popper and D. F. Rushman, Nature, 160,58 (1947). (5) W. Kiinzig and R. Meier, Xclv. Phys. Acta, 2 2 , 585 (1948). (6) S.Sawada and G. Shirane, J . Phys. SOC.Japan, 4 , 52 (1949).
TABLE I HEATCONTENTS ABOVE 298.16'K. (CALJMOLE) HT HT HT -
T,'K.
Hte8.1s
T,"K.
R1P.16
T,OK.
H~9s.ls
355.6 372.4 386.5 408.3 463.7 557.5 659.2
BaTiOs (mol. wt. 233.26) 1,450 767.3 13,150 1293.3 1,920 869.5 16,180 1364.6 2,310 872.5 16,340 1464.3 2,920 987.9 19,820 1590.3 4,160 1091.0 23,000 1707.5 7,070 1198.4 26,360 1831.1 10,020 1287.5 29,170
29,290 31,520 34,670 38,770 42,520 46,560
384.0 463.0 553.6 652.8 762.3 866.2
SrTiOt (mol. 2,110 985.5 4,170 1089.8 6,620 1200.3 9,440 1292.6 12,550 1342.1 15,560
33,100 35,850 38,630 41,550 44,790
397.5 503.1 594.5 690.3 805.3 807.1
BazTiOc (mol. wt. 386.62) 3,670 922.1 26,030 1432.2 7,830 1020.4 30,320 1539.4 11,620 1120.7 34,720 1631.3 15,730 1231.8 39,620 1736.7 20,780 1335.3 44,210 1831.3 20,830
391.8 490.3 610.7 719.5 830.3
Sr2Ti04 (mol. wt. 287.16) 3,290 938.2 25,110 1432.1 7,000 1039.5 29,320 1510.1 11,670 1141.9 33,610 1628.2 16,120 1265.6 39,030 1726.0 20,610 1361.3 43,360 1820.7
wt. 183.53) 19,130 1448.7 22,310 1538.9 25,610 1630.7 28,380 1727.9 29,920 1831.5
48,490 53,340 57,420 62,240 66,830
46,330 49,700 54,700 58,930 63,320
400°K. Examination of the heat content data for this effect necessitates use of some differential method of plotting, such as that in Fig. 1 in which there is plotted
against T. This function has a cusp-like maximum a t about 400"K., which indicates the existence of the Curie-point for barium metatitanate and confirms the magnitude of the heat effect noted by Blattner and Merz. The same figure also shows the analogous function for the barium-strontium orthotitanate combination. Except for the work cited for barium metatitanate, there are no previously reported high temperature heat content or heat capacity data for these substances. Statton's' melting point study of the (7) W. 0. Statton, J. Chcm. Phys., 19, 33 (1951).
Feb. 5, 1953
HEATCONTENTS OF BARIUM AND STRONTIUM META-AND ORTHOTITANATES
53 1
TABLE I1 HEATCONTENTS (CALJMOLE)AND ENTROPIES (CAL./DEG.MOLE)ABOVE 298.16'K. 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800
7.76 13.90 19.08 23.56 27.51 31.06 34.26 37.18 39.88 42.39 44.72 46.92 48.98 50.94 52.81
2,695 5,450 8,290 11,200 14,160 17,170 20,210 23,280 26,380 29,510 32,660 35,840 39,040 42,270 45,540
2,530 5,170 7,920 10,750 13,640 16,580 19,560 22,560 25,570 28,590 31,620 34,660 37,700 40,750 43,830
7.28 13.17 18.18 22.54 26.40 29.86 33.00 35.86 38.48 40.90 43.14 45.24 47.20 49.05 50.81
BaO-Ti02 system gives 1970 and 1960'K. as the melting points of barium meta- and orthotitanates. The measured heat content results are represented (to within the average limits of error given in parentheses) by the following equations, which were derived by the method of Shomates: BaTiOa (metatitanate): H298.16 = 29.03T T-1 SrTiOs (metatitanate) : HT - H29s.1.3 = 28.23T
HT -
+ 1.02 x
- 10,282
+
lo-* 4.58 x lob (298-1800'K.; 0.3%)
+ 0.88 X lo-* P + 4.66 X 10' - 10,058 (298-1800°K., 0.3%)
T-'
3,780 7,730 11,860 16,140 20 ,520 24,950 29,380 33,810 38,240 42,670 47,110 51,570 56,060 60,590 65,170
3
10.88 19.69 27.22 33.81 39.66 44.88 49.55 53.77 57.62 61.17 64.46 67.52 70.42 73.17 75.79
-
-
+
-
+
Table I1 contains heat content and entropy increments above 298.16'K. a t even 100°-values of (8) C. H. Shomate, THIS JOURNAL, 66, 928 (1944).
IO.40 18.78 25.89 32.07 37.53 42.42 46.84 50.88
54.60 58.04 61.24 64.23 67.03 69.67 72.16
3
1 3 $1
a> .a 8 0 8
6
8 '9 1 $ a
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800 m
BazTiOl (orthotitanate) : + 6.96 HT H298.16 = 43.00T + o.80 T-1 15,226 (298-180OoK., 0.6%) SrzTiOa (orthotitanate): HT Hss.1~= 38.45T 1.92 X lo-$ P 4.67 X IO6 T-1 13,201 (298-180OoK., 0.4%)
3,610 7,370 11,270 15,280 19,370 23,520 27,720 31,960 36,230 40,530 44,850 49,180 53,520 57,870 62,230
1200
1600
"77
-n. Fig. 1.-Differenceinmeanheatcapacity: curveA, BaTiOsSrTiOl, curve B, BaaTiO&rtTiO,. I',
temperature, for use by those who make thermodynamic calculations in tabular form. The entropy increments were derived, so as to match the heat contents, by the method of K e l l e ~ . ~ BERKELEY4, CAL~XNIA (9) K. K. Kelley, U.S. Bur. Mines Bull. 476 (1949).
