Emet kolemanitinin ferroelektrik özellikleri

Abstract

İl ÖZET Bu çalışmada Emet kolemanit yataklarından temin ettiğimiz kolemanit tek, kristalinin tabii yüzlerine ait Miller indisleri, iki daireli gonyometre kullanılarak saptandı. Neticede, kolayca yarılabilen' yüzdün (010) yüzü olduğu görüldü. Geniş (010) yüzlü levhalar altın buharıyla kaplanarak, sığası pF mertebesinde olan kondansatörler yapıldı. Bu kondansatörlerin sığa ve kayıp tanjantları LCR köprüleri ile, 100 Hz-2 MHz frekans aralığında, sabit ve düzgün değişen sıcaklıklarda ölçüldü., Sıcak lık değişimleri için alkollü kryostat ve sıvı azotlu soğutma sistemi kullanıldı. Perroelektrik 'kolemanitin spontan polarizasyonu Sawyer-!]* ower köprüsüyle ölçüldü.; Bu köprüde jeneratör,.bipolar voltaj yükselticisi, osiloskop ve 3MT kaydedicisi kullanıldı.Ill Yapılan deneyler neticesinde, (01Q) yüzüne dik b ekseni boyunca Emet kolemanitinin £ dielektrik sabitinin ve t g. 6 kayıp tanjantının -3 °C ve -18 °C yakınlarında tepeler oluşturduğu, bu noktalarda faz değişikliğinin olduğu saptandı. Kölemanitte bu tip faz geçişi ilk defa tarafımızdan gözlenmiş oldu. o Spontan polarizasyon ölçümlerinden ise -18 C nin altındaki fazın ferroelektrik faz olduğu görüldü. -3 C nin üzerindeki fazın paraelektrik faz olması yanında, -3 C ile -18 C arasında görülen fazın *; v* Emet kolemanitinin bir özelliği olduğu bulundu. Paraelektrik fazda ters dielektrik sabitinin sıcaklıkla lineer olarak değiştiğini gözledik. Bu doğrunun eğiminden Hesaplanan 350 C lik Curie sabitinin literatürdeki değerden 100 C daha düşük olduğu belirlendi. -3 C nin üzerindeki paraelektrik bölge ile -3 °C nin altındaki bölgede ters dielektrik sabitinin sıcaklığa göre değişim eğrilerinin eğimlerinin oranının -2.8 ile -3.2 arasında olduğu gözlendi, (100) yüzünün altınla kaplanmasından elde edilen kondansatörlerle yapılan ölçmelerden, [lOOj doğrul tusundaki dielektrik sabitinin -3 C ile -18 C de çok az değiştiği, -18 C den sonra yavaşça azaldığıIV Emet kolemanitinin -18 °C nin altında ferroelektrik fazda 8000 Volt/cm değerinde iç bias elektrik alanının olduğu saptandı.. -50 C de yapılan ölçümlerde görülen spontan polarizasyonun 0.65 2 uCoulomb/cm değerinin, literatürdeki değerlerle uyum içinde olduğu bulundu. 2 1

V FERROELECTRIC PROPERTIES OF EMET COLEMANITE.SUMMARY - ? Colemahite iş a natural ferroelectric crystal. On its ferroelectric properties many researches have been made since its ferroelectricity was discovered by Goldsmith in 1956 (4), At room temperature cole- manite is nonpolar with a space group P2,/a (6),. but below approximately -3 C it. transforms into a phase.which is piezö-, pyro-, and ferroelectric. The polar phase has been shown to be `.3*2_ (9). -.'??'- ?.*?? : For the paraelectric-f err oeleotric phase transition temperatures, different researchers have recorded different values. From pyroelectric measure ments i Davisson (3) and Chynoweth (17) found it to be -2.5 °C and -6 °C, respect ively. The value recorded by Wieder (14) was -7 C. In his intrinsic. bias field investigations Fatuzzo (10 ) gave the value -2. 5, C for the trans it i on' t emperatur e. Holuj and Pet ch ( 9 ) found one or two consecutive transitions between 0 CVI and -5 C in their nuclear magnetic resonance studies. Theveneau and Papon (22) recognized the ferroelectric phase transition at -1.70 °C in their nuclear spi*ı-lat t ice relaxation time measurements. Besides this uncertainity at the paraelectric- ferroelectric phase transition temperature, Wieder et. al. (20) observed that some colemanite specimens (Erd Colemanite) showed dielectric anomalies at -12 °C and -30 C. The synthetic policrystalline pallets prepared by Wieder et al. also showed a phase transi tion around -33 0. These differences related to the ferroelectric transitions in colemanite initiated us to study with colemanite samples obtained from Western Anatolian borate deposits. There are nümerious large borate deposits in Western Anatolia, in Emet, Bigadiç, Faraş, Kepsüt, Susurluk, Kestelek, and Kırka (2). Inmost of these deposits colemanite iş the main ore, especially in Emet. In tlie present study we used the colemanite crystals taken from Emet. Using an optical goniometer Miller Indices of the neutral faces of colemanite crystals were found, and it was seen that the faces which were easily cleaved were (010) faces.VII Flakes with, large (010) faces were coated with gold by evaporation technique, and capacitors with the range of a few picofarad were obtained. The capacities and the loss tangents of these capacitors were measured by LCR bridges in the frequency range of 100 Hz-2 -MHz and the temperature range of..-180' C and +20 C» Dielectric measurements exhibited two peaks for both the real and the imaginary parts of the complex dielectric function in the vicinity of -3 C arid -18 C. These peaks are the characteristics of the phase changes of Emet colemanite. Through spontaneous polarization measurements it was found that the phase below -18 C is ferroelect ric. It is known from literature that above -3 0 colemanite is in pafaelectric phase. The phase between -18 C and -3 0 was interpreted as a diffuse Curie range, and it may be due to the arsenic impurity which is found only in Emet colemanite samples at the level of 20 ppm. In the paraelectric region the inverse dielect ric constant changed linearly with temperature; the Curie constant evaluated from the slope of. this line is around 350 °C. The ratio of the slopes of the linesVIII from -8 ° C to -3 °C to those in the paraelectric region were found between -2.c8 and -3.2. In the ferroelectric region below -18 °C an internal bias of 8000 Volt/cm was observed. It was.almost constant. At -50 °C the spontaneous polari zation of 0.65 uCoulomb/cm was' measured. The dielectric constant measurements along nonpolar [lOOJ direction didn't exhibit any sharp peak at - 3 °C and -18 °C. Four assumptions are prop'osed in order to -explain the two peaks observed in the dielectric measurements, of. 'JSmet eolemanite:.1. `Natural eolemanite. contains' impurities, such as,.; Fe.O.y SrO, HgO, Ş.İ0.,, A1.0, K_0 and As^yv' Tlies e impurities,?. eş pec ially 'As. 0... which is found only in linet district, : causes inhomogeneouS; region, through out; the crystal ?between.-3?C and -18°0..?'-?''?:. In these.microregions,: Curie.' constants may be differ ent as it was proponed for oorae `a^iO -Tw^nO-. 3 : $ ' r. olid sol it ions (33) 2* Second approach is similar to the approach of.'.Yiede.r. et al. for ürd eolemanite (20). htaetIX eolemanite is composed of two isomers. One of them has a ferroelectric phase below ~1 8 C and above this temperature it is paraelectric. Second isomer has a nonfe.rroelectric phase transition at -30C. 3. fhe eolemanite samples may exhibit a paraelectric-* ferroelectric phase transition at -3 °C and ant if err o- electric-f err oelectric phase transition at -1 8 °CV 4« From the structure studies it.is: kno/m tiiat : eolemanite has two metaş table hydroxide group. Öne...of.'..them belongs to the crystal water and the ?' ^.2 other is the one of the hydroxide in the Ba_0, (QH)J radical (5,12), It may be the case that one of these bistable hydroxides becomes stable around ~3°Q / and does not initiate the ferroelectricaty: in eolemanite, whereas: other hydroxide becomes, stable '?? k. at -18 °0 and causes the ferroelöctricity in eolemanite*