BY SUSAN TROLIER-McKINSTRY, HAROLD M. FROST, AND CLIVE A. RANDALL
SUBMITTED BY THE NAE HOME SECRETARY

LESLIE ERIC CROSS passed away peacefully December 29, 2016, at the age of 93. He was a world leader in the field of ­ferroelectrics from a fundamental perspective, as an inventor of new characterization techniques and materials applications. During his long and robust scientific career, he was beloved for his intelligence, vision, wit, and humanity, as well as the charm with which he shared his fascination with ferroelectrics and his newest ideas. He was also an excellent mentor, and many of his students and postdoctoral researchers went on to scientific leadership positions themselves.

He was born August 14, 1923, in Morley, West Yorkshire, England, to Charles Eric Simeon and Alice Emily (Plant) Cross. He studied at Leeds University, where he received his BSc in physics, with honors, in 1948 and his PhD in ferroelectricity in 1952.

World War II temporarily interrupted his undergraduate education and he worked for the British Admiralty on a program using high-frequency direction finding to track German U-boats, which ultimately allowed Allied convoys to cross the Atlantic unharmed. Just 2 weeks after his transfer to that assignment, the boat he had served on was sunk in the Humber estuary in northeastern England, with no survivors. Ever after, he thought of himself as a lucky man.

He began his research career studying dielectric and ferro-electric materials at the British Electrical Research Association. In 1961 he immigrated to the United States to accept a position at Pennsylvania State University as a senior research associate. He rose through the ranks and in 1985 was named Evan Pugh Professor of Electrical Engineering—the professorship is the highest distinction that the university can bestow on a faculty member.

His earliest published research was in characterization of the properties of BaTiO3, including both the optical response and switchable polarization. He made some of the first correct measurements of spontaneous polarization, substantially higher than previous data, as he had achieved more complete switching. Indeed, it was not until his work was reproduced at Bell Labs that his rejected paper on the subject was accepted for publication, with apologies from the editors. He later extended this work to NaNbO3 and KxNa1−xNbO3 (now under intense investigation as a lead-free piezoelectric material). This appears to have been his introduction to antiferroelectricity.

At Penn State he developed phenomenological modeling for understanding property correlation in ferroelectric ­materials. He was interested, in particular, in polarization and strain ­coupling through electrostriction; this approach influenced a large body of his work as well as numerous subfields in ferro­ electric and related materials. His pioneering work spanned the application of ferroelectrics in bulk ceramics, ­composites, single crystals, multilayer technology, and thin films. He also developed an early model for the grain size effect on the ­dielectric properties of BaTiO3 and studied improper ferro-electric materials­ such as gadolinium molybdate. And he is the only person we know who built his own oscilloscopes as needed to make measurements. 

In the 1970s and ’80s, with longtime colleague and close friend Robert E. Newnham, he developed piezoelectric composites and laid out the symmetry requirements for secondary ferroics. The composites work was originally motivated by his work on sonar systems, but ultimately led to major improvements in ultrasonic imaging transducers and is now ubiquitous in medical ultrasound systems.

He conducted key measurements on the contributions of domain walls to the properties of dielectric and ­piezoelectric responses, and began an investigation into the origins of dielectric dispersion in ferroic materials, which eventually led to major discoveries in the role of chemical order-disorder in relaxor ferroelectric materials as well as the dynamics of the nanopolar regions.

He and his students also laid out the phenomenology of the lead zirconate titanate system, an essential means of describing the intrinsic properties of materials in the absence of single crystals. This work is still widely used and cited.¹

His drive to improve means of measuring strain and electromechanical coupling under a wide variety of temperature, frequency, and field conditions led to the development of numerous new measurement methods, including the use of a double beam laser interferometer.

From 1990 to 2014 he coauthored papers identifying the origin of bridging phases near morphotropic phase ­boundaries in ferroelectric solid solutions, contributed to the understanding of domain-engineered piezoelectric single crystals, designed new piezoelectric transducers and magnetoelectric com­posites, solidified understanding of domain wall contributions to the nonlinear behavior of ferroelectrics, and was one of the first to exploit ferroelectric thin films for piezoelectrics in microelectr­ omechanical systems (MEMS). His final key contribution to the field was the discovery of anomalously large flexo-electric coefficients in many perovskites such as Ba_1−xSr_xTiO₃.

He authored or coauthored some 850 refereed papers, held 20 patents, and coauthored a comprehensive textbook, Domains in Ferroic Crystals and Thin Films, with Alexander Tagansev and Jan Fousek (Springer, 2010). And as a professor and mentor at Penn State, he mentored over 50 graduate students from around the world, including Yao Xi, the first Chinese PhD (1982) educated in the United States after the Cultural Revolution.

In addition, through the lifelong friendship at Penn State of “the gang of four”—the other three were Newnham, Rustum Roy, and Della Roy—Eric helped to build a culture of coopera­ tion and collegiality that went far in establishing the Penn State Materials Research Laboratory as a preeminent, inter­ disciplinary research facility, a forerunner of the Materials Research Institute, of which he was a founding member.

In short, Eric contributed to the understanding and application of virtually every major ferroelectric material. He came to the field of ferroelectricity in its infancy, to the objection of his advisor Edmund C. Stoner, who referred to it as “a trivial lattice phenomenon!” Cross laughed many years later that he was still trying to understand this trivial phenomenon.

He also worked with companies all over the world on ferro­ electrics for capacitors, piezoelectrics, pyroelectrics, ­dielectric bolometers, tunable microwave devices, and electrooptic applications.

His work had a worldwide impact on industrial research, development, and manufacturing. His understanding of ferro-electric phases and their domain contribution in piezoelectric ceramics enabled highly sensitive underwater sonar devices and medical ultrasound imaging. His group was one of the pioneers in the fabrication of ferroelectric materials in thin film form to enable the field of piezoelectric MEMS. He also developed the double beam interferometer that enabled electromechanical strains to be determined quantitatively on thin films. In the latter years of his career he looked for alternative methods to create sensing materials, and he explored the use of a once obscure strain gradient phenomenon known as flexoelectricity. He developed key measurement technologies, identified materials with anomalously large coefficients, and engineered a number of devices around this concept.

He had a keen interest in practical solutions to problems. When the Hubble Space Telescope was found to have been launched with incorrectly ground mirrors, he worked closely with engineers to select the electrostrictive adjusters that restored the telescope to functionality.

He was very proud of his long association with the Department of Defense and particularly the US Navy, which supported much of his work in the field of sonar undersea transducers. One of his colleagues on the DARPA Materials Research Council, Arthur Heuer, of Case Western Reserve University, called Cross a national treasure.

Professor Cross was honored by many professional organizations. In addition to his election as an NAE member, he was a fellow of the Materials Research Society, American Physical Society, Optical Society of America, American Ceramics Society, and Institute of Electrical and Electronics Engineers. In 2010 he received the Von Hippel Award of the Materials Research Society, its highest honor.

He shared his ideas freely with everyone he met at scientific conferences, from graduate students to senior leaders in the field. With his trademark exclamation of “Jolly good!,” his mischievous humor, and his tendency to wear socks and ­sandals with suits, he will be sadly missed by all who knew him and worked with him.

Eric loved his work, but equally important was his love for his wife of 61 years, Lorna Lucilla (Cilla). Cilla excelled at the social side of the ferroelectrics community, opening their long-time College Heights home to generations of students and colleagues and occasionally their dogs.

He was a kind, supportive father, grandfather, and great-grandfather. He carved a turkey like a surgeon, could jump-start his kids’ stranded cars at all hours, and built a giant backyard play-fort complete with intercom and electricity. For his six children, Christmas was joyful chaos and summer holidays were a parallel universe of woods and beaches punctuated with his classic phraseology.

Cilla died in 2011. They are survived by their children Matthew and his wife, Holly, of Cypress, Texas; Daniel and his wife, Catherine, of Alexandria, Virginia; Rebecca Cross, of State College; Rachel Jennings of Manassas, VA; and Elizabeth Cross, of Topanga, California; six grandchildren; and five great-grandchildren. Their son Peter died in 2004 and is survived by his wife, Pam.

The scholarship, passion, and personality of Professor Cross can be appreciated in a 2011 interview posted at IEEE’s Engineering and Technology History Wiki (http://ethw.org/ Oral-History:L._Eric_Cross).

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Adapted from a tribute posted on the PSU Materials Research Institute website (https://www.mri.psu.edu/mri/news/memory-l-eric-cross). The authors also greatly appreciate input from the Cross family and from Walter Ernest Mills.

1  He coauthored a series of five articles on different aspects of “Thermodynamic Theory of the Lead Zirconate-Titanate Solid-Solution System,” published in 1989 in Ferroelectrics 99, pp. 13–86.